I've heard that the Pokémon TCG cards from the old sets, like base, jungle, fossile etc, aren't allowed in tournaments anymore. Why are they forbidden? The old cards seem to be consistently weaker, so it can't be a balance issue. Are there incompatibilities between the rules?
Also: Which sets are forbidden?
There are a good collection of Pokemon TCG tournament rules and resources here - you may find some of it useful.
Looking at the rules for tournament formats, I see that there is a format called "Modified" that restricts players to using more recent sets - but there is also an "Unlimited" format where any cards ever printed are fair game. So, in theory, there's no reason why you couldn't find an Unlimited tournament to play your older cards in. Lobby your local tournament organizers to run one, if they aren't doing so already!
Why would people prefer to run Modified tournaments? Well, as Aramis correctly states above, the manufacturers have a vested interest in people buying new cards in order to stay competitive. However, it's not all for evil, economic reasons. It would be deeply unfair to new players if, every time they turned up at a tournament, they were crushed by long-time players with vast collections of powerful, out-of-print cards. It's not necessary for the old cards to be stronger than the new cards for there to be a problem: the players who have been collecting for a decade will have so many more options to choose from that by definition they will be able to build better decks. And if an older, out-of-print card is actively unbalanced, then it's very unfair to ask new players to spend big bucks getting hold of these, just to compete.
Compare and contrast to Magic the Gathering: there is a format called "Vintage" which is quite popular - but to build a truly competitive Vintage deck you need cards like Black Lotus and Moxes, which cost hundreds of dollars apiece! If all Magic tournaments were Vintage, then only players who have been playing for 20 years or have tens of thousands of dollars to spend would stand a chance of winning a game. And nobody wants that!
Whilst I don't play them anymore, CCG sets have many reasons to block older cards
While play balance is the big one, opponent parity and loopholes are related, it's also not one the Pokémon players complain about. Some old cards are over the top, and many not up to par; sets can't effectively be playtested against all older sets, so balance can't be maintained against them.
The most obvious is the economics: if you could continue to play your old cards, you have less need of new ones (unless the power creep is too high). By requiring only the last couple sets, it both forces old players to buy new cards, and enables new players to get in without having to spend huge amounts on older cards.
Many times, a new edition also alters some rule. This can make older cards special abilities not so special, or render an otherwise balanced card way out of proportion. Since Tournament play almost always uses the newest rulebook only, the older the card, the more likely to be thrown out of balance by subtle rules changes.
Loopholes are seen by some as a balance issue... but really, they are separate. Certain cards in some games work fine, but interact with certain other cards in strange ways. There are three solutions: special case rules in the rulebook, special case rules in the FAQ, or disallow the older card. It's easier just to disallow the older of the cards.
Player parity: if older sets are stronger, new players wouldn't be competitive. If the new sets are stronger, older players wouldn't be. But a Tournament is supposed to be about skill, not who can afford the best deck... so by limiting the choices, it prevents opponents from being pushovers, as the most recent few sets are likely to be closest in power levels.
Pokémon is, according to the students I've overheard, pretty well balanced even against the older cards, so I suspect most of it is parity and economics.
Tam, I'm way late to the party but in case someone else finds this by Googling an answer for a similar (or in my case pretty different =P) question, I'll answer it anyway.
I don't know how long you studied the Pokémon TCG before you made your statement about older cards being weaker. Even taking into account that I am making this comment in 2016 while the question was asked back in 2011, what I am saying was true back then. For the record, I began playing the Pokémon TCG in 2011, with its North American release, give or take a break when school or work got to be too much. Even then I would research what I missed.
So I'm telling you some of the most powerful cards in the Pokémon TCG are from the original "Base Set". The catch is that they aren't Pokémon, but Trainer cards! There are some Pokémon and Energy cards that would cause problems as well because their effects (not their HP or attack damage) is so powerful, though at the time the game split between Unlimited (all sets allowed) and Modified (the latest sets allowed, rotating older ones out once per year), the older Pokémon were also stronger than most of what was being printed. For the record, Modified is now referred to as "Standard".
Additional reasons, besides a company needing to sell new cards to make a profit and stay in business, are core rules being revised and altering how good older effects are and the complexity of an ever growing card pool. Do you know how many total Pokémon cards have been released in English? I believe with the most recent set it is really is "...over 9000!". A lot lower when we ignore reprints, but even just looking at unique cards it is probably at least a few thousand.
A few thousand cards is a lot for a new player to learn, to sort out the best version of a particular Pokémon that may have released 10 years ago but is no longer available. It is also a lot for a designer to check his latest creation against to make sure it isn't too powerful via combo. Almost no one plays competitive Unlimited anymore and around the time you asked your question, it was still true. Decks in Unlimited where either First Turn Win or First Turn Lock; you could actually lose or effectively lose before you got to take a single turn.
And all this is just the general; this would be much longer if I were to give specific examples, though if they are needed I can. ^_^
