I notice that hotshoe flashes (e.g. Nikon SB-800) typically have a Fresnel lens system to focus the light, but studio strobes (e.g. Photogenic Powerlight 1250) do not. What's up with that?
Fresnel spots are one of the traditional studio-lighting tools, carried over from theatre and movie work; they allow focusing the light from wide to narrow, just like the zoom function on a flashgun. For example this one from Elinchrom, or these Broncolors.
The reason you don't see them often is that, like a scrim, snoot, honeycomb, or other tools, they're a fairly special-purpose device that gives a particular look; one you don't want all the time. They were widely used in the 30s-50s in Hollywood movies and studio photography, which today gives them a fairly 'retro' look when used on their own, but they're still around (and are still the workhorse lights of theatre and movie production). I'm not a particular student of lighting trends, but I'd describe them as slightly harder light than is currently fashionable for portraiture.
On the other hand, flashguns have a couple of special properties that make a fresnel lens attractive for that application. A big one is effciency: Since power for flashguns comes from batteries, this is a much bigger concern than for mains-powered studio lights. The fresnel helps to focus the light to the desired field of view, and so the same power can be used to produce a higher GN (or extend battery life with the same GN).
Fresnel lens are rarely used in studio strobes because in studio we mostly need soft light, not the hard one. And if we need hard light, we can always use standard reflector. And if we need spot light, we can always use tube reflector.
Fresnel are used on hot-shoe light because there are different focal lengths of different lens, the hot-shoe light is located at the fixed position (at the camera) and must use every single ray to light the scene and not spill around. In studio you can move the light whereever you want.
Given all that fresnel lens in studio are almost useless. (However they do exist.)
I can't say for sure, but my spontaneous guess would be a tradeoff between portability and quality. The fresnel lens takes less space and weighs less than a traditional lens, so it's more practical in the case of a hotshoe flash.
On the other hand the fresnel lens typically has a lower image quality than traditional lenses. Perhaps this affects also the quality of the projected light in some way that is undesirable, which could explain why they are still in use in studio strobes, where portability is a less important factor.