The major reason was prevent the formation of a large, politically dangerous standing army. The experience of England, then Britain, as compared to the countries of the continent in the previous 200 years had demonstrated that if a monarch had a standing army, he could disregard the will of the people and make himself an absolute autocrat/dictator.
All the major kingdoms on the Continent, France, Spain, all the Germanies etc once had representative bodies like the English parliament. One by one they fell to the power of the king and his standing army. With a standing army, the key could just take without consent.
England dodged that bullet by being an island. An Island creates special defense problems, particularly back in the Age of Sail:
- Ships were the fastest form of transport and a seaborne attack in one area could outrun the warning to another area.
- Ships could switch their point of attack if repelled or the weather turned or for other reasons.
- Ships could quickly mass at any given point on the coastline more man and fire power than any land force. If a serious attack occurred, the local resources that were on immediately on hand (like a fort) or with a couple of hours of travel would be all that could used to repel the attack.
All these factors played a large part in the successive waves of invasion prior to 1066. Although no one succeeded in landing a large force after the Normans, there were dozens of near successes that failed owing to weather, the actions of the British Navy or the enemy fear of that navy. Ireland was invaded several times by the French. Once the evaded the English Channel fleet, they had a smooth run.
Clearly, the only land force that could actively defend in that environment would to arm the general population so they could resist landings immediately. In the mid-1800s, they even created a militia of gunboats kept socked away near but usually not in or on likely landing areas.
America faced the same problem of having a huge area and boundary to defend in a time when it took weeks to travel from New York to Philadelphia. Any attack would have long failed or succeeded before Washington even knew the attack had occurred. Consequently, a coastal defense navy, a small standing army defending choke points and providing an officer core around which an larger army could be built in a real war, combined with a militia (which meant at least all enfranchised males) was the only conceivable defense strategy.
But perhaps more importantly, the Founders knew of the circumstances of English Civil War. It began with King Charles trying to like a sole autocrat who used imprisonment, torture and executions to enforce his breaking of law after law.
The war itself proved more than a little farcical, at least in the beginning, because the only Englishmen with landwar experience were those who had gone to fight on the continent. Most English cities still had medieval defense works which had not been updated to the new earthern star forts.
But with the political and social structure disrupted, a higher degree of meritocracy grew within the Parliament's side especially among the the religious non-conformist. With a few years, the New Model Army evolved into the world's most effective standing land army...
...and promptly took over the government. Just as many had history of England and the Continental experience warned, a standing army could crush the Constitution and Liberties of any land. If Oliver Cromwell had been a different sort of man, he could have murdered the English representative democracy and England would have continued to be dictatorship with parliament fading away to a mere ritualistic body just as it did in France. Fortunately for the rest of Anglosphere history, he spend most of political capital trying to find a way to restore the Parliament and restore representation.
But everyone in England had learned the lesson of the political dangers of standing army.
After Cromwell, with Parliament restored and the weasel Stewarts back on the throne, the first thing the Parliament did was to pack the new Model Army off to Ireland, Tangiers and various colonies. They did so to prevent the Stewarts from using them like Cromwell had. It clear that if Charles II would not have that James the II would.
This was the political history lesson that the Founders learned: A standing army will eventually lead to dictatorship. The lessons of the revolution itself seemed to prove it as the Revolutionary war pitted the rebels against professional soldiers from Britain and King George's German lands (the Hessins.) It took little imagination to project that lesson onto America itself with the President or a General using a large standing Army to overthrow democracy.
Battles like King's hill and the First Saratoga demonstrated that civilians equipped with civilian hunting rifles could harry and sometimes defeat trained soldiers armed with military muskets. The difference being virtually same between a modern deer rifle and a military assault rifle i.e. long-range, accurate but slow fire, and with poor close contact performance vs short-range, in-accurate but fast firing with good close contact performance i.e. bayonets.
The Founders had just come out two decade long struggle in which they saw a once trusted government suddenly begin to violate long established rights, then deploy professional soldiery against the citizens who could only begin to exist because they had large numbers of private weapons on hand.
Put plainly, without private weapons, there would have been no Revolution, not Constitution and no United States.
There was ever any serious debate in any of the States about whether citizens had a natural right to be armed. The major constitutional debate centered on what powers were to be granted to the Federal and State Governments to take control of the militia. Remember, this is time when you could walk into a store and by ship and field artillery with no paperwork. Merchant ships had to be armed and any private or locally managed fortifications needed cannons as well. Plus we had this vast frontier where people were always on their own.
To be frank, there is a lot of flat out lying from people who question original intent in the 2nd amendment and other parts of the Constitution. They try to talk about it like it some sacred text handed down out of the dawning mist of time from some unknown source and therefore requires massive interpretation to make some sense out of the whole thing.
That's a lie. The Constitution is just 240 years old, not 2400 years. More importantly, it is likely the most widely debated and voted on political document ever produced by any polity in the whole of history. Every single word or phrase was thrashed out, in public, for months or years before the people voted on it. In the case of the 2nd Amendment, every phrase in it had a concrete definition, widely understood by the ratifying public and there is no ambiguity at all in the 2nd Amendment. Those who say that the constitution requires some near mystical interpretation by modern priest class are lying.
Of course, that doesn't really matter to they post-modern Judiciary. Consider the legal term of art in the 8th Amendment:
Excessive bail shall not be required, nor excessive fines imposed, nor cruel and unusual punishment inflicted.
The phrase, "cruel and unusual" had a iron clad definition in English law for more than two centuries at the time. A "cruel" punishment was one disproportionate to the crime itself e.g. life imprisonment for a parking ticket, while the "unusual" meant that people who committed the same crime should receive the same punishment. The reason for the phrases use was to prevent the government from singling out individuals it disliked for disproportionate punishment that they inflicted on no one else.
In no way did the phrase invite philosophical debate about the nature of punishment itself. The phrase is there solely to protect specific individuals in specific cases for being treated differently.
But that's what the court did in Furman v. Georgia which halted executions in the US resulting in dozens of innocent deaths from the freeing of those once headed for execution (Kenneth McDuff in Texas being a good example.)
Once someone starts trying to debate whether the death penalty itself is at issue in the 8th Amendment, they're lying about amendment. And no there is no room for debate on the facts. You can spend literally years reading all the debates that lead up to the ratification of the U.S. Constitution. We know exactly with no ambiguity what the every jot and tittle meant when the people gave their "consent of the governed" when they ratified it.
People who argue otherwise are elitist who a tiny minority to be able to bend, warp and outright destroy any part of the Constitution they wish on their least whim with no restraints. The Constitution has a built in mechanism for its change and evolution, the elitist just can't use it because deep down, they have no respect for the people and are so impressed by their own brilliance they can't imagine they need anyone else's input. They don't want to spend the years it takes to get everyone to agree to a new amendment, they just want to change it as they wish, when they wish.
These are the people the Constitution was designed to stop. We have the 2nd Amendment precisely because the Founding generation foresaw exactly this circumstance. We've had 240 years to change it if majority thought we needed to and we have not done so.
Judicial fiat does not represent the "consent of the governed" and we have within a hair's breath of sliding into a Judicial Depotism in which the law is whatever the judges say it is, not whether what the people voted on.
It's getting scary.