While a common zoom lens might be 24-70mm and there are super zooms with ranges like 24-240mm I have yet to find a lens that starts in ultra wide angle and goes up to normal focal range. For example a 15-50mm (35mm equivalent) lens.
Is there any reason beyond expected customer demand not to build such a lens? Is it in some way harder to have a long focal range that includes ultra wide angles?
Why no wide angle to normal length super zooms?
In order to make ultra wide angle lenses, a retrofocal group is usually used. The retrofocal group is essentially a reversed telephoto at the back of the lens.
Now, to transition from the ultra wide angle region into wide and normal fields of view, we'd have to eliminate the retrofocal group from the optical path. That's not impossible, but it requires "switching out" the retrofocal group, similar to some lenses that have built-in teleconverters that can the switched on or off (such as the Canon EF 200-400mm f/4L IS USM Extender 1.4x, or Nikon's recently announced AF-S Nikkor 180-400mm f/4E TC1.4 FL ED VR).
In addition to removing the retrofocal group, the wide field-of-view collecting concave front elements (the fisheye elements) need to be removed or "switched out".
Conversely, to get into the superzoom range, you'd definitely need a telephoto group in front of the lens. But similar to the retrofocal group, you want the telephoto group to be removed from the optical path when you're working in the wide angle regime. Because of the need for a useful maximum aperture, the telephoto group has to be large, much larger than the retrofocal group. In rough terms, the glass has to be much bigger at the front of the lens than at the rear. So a switchable telephoto group needs a compartment to store the group in the lens, but out of the optical path. It would look like a goiter at the front of the lens.
When it comes to designing optical formulas, the components of a lens aren't designed in complete isolation of each other — decisions made in one group of optics impact decisions that need to be made in other groups of optics. So if you don't design the main group(s) and focus group(s) with the retrofcal group in mind, then the ultra wide angle performance will suffer. If you don't design the main and focus groups with telephoto performance in mind, then the telephoto performance will suffer. But by needing to make each of those groups removable or switchable, you're requiring mediocre performance at the very best, while also saddling the lens with huge size and weight penalties. And likely prohibitively expensive. And the lens would probably be outperformed by a small group of 2 or 3 kit lenses covering the same focal length range.
On the marketing side of things:
Most people who shoot full frame tend to prefer lenses that do only one thing very well or a few things fairly well rather than a lens that can do a lot of things at a mediocre level. So wide angle zoom lenses for FF tend to have a more limited focal length range to allow for that higher image quality. The Canon EF 11-24mm f/4 L or the Nikon AF-S 14-24mm f/2.8 G are both examples of such lenses.
On the lens design side of things:
Very wide angle lenses made for formats/mounts with registration distances considerably longer than the focal length of the lens get increasingly difficult and expensive to make. This is particularly the case when wider maximum apertures are desired. Most of the shooters who might be interested in a 15-50mm f/2.8 FF lens would likely be put off by the price required for the optical performance such a lens would give.
Putting both lens design and market conditions together:
Until the recent release of the third generation Canon 16-35mm f/2.8 L the image quality was not exactly stellar and only those who really needed f/2.8 with a zoom lens in that focal length range bought it (mostly very low light wedding/event shooters). The new EF 16-35mm f/2.8 L III gives much better optical performance than its predecessors, and the price of the lens reflects that ($2,200 USD MSRP and $2,000 USD Street compared to the "II" version that sold for around $1,400 USD). Those who don't need the f/2.8 aperture can get a sharper lens and image stabilization at a much lower price ($1,000 USD) with the EF 16-35mm f/4 L IS.
This is also more or less the case with other lens makers. The wider the focal length range, and/or the wider the maximum aperture, the lower the overall image quality the lens delivers even at a higher price than a lens with a narrower focal length range or maximum aperture. This is particularly the case in ultra-wide angle territory.
