I teach a class at a gym. I rarely do more advanced poses for a few reasons:

-There are *always* beginners in the class.
-I've found that, as much as I provide instruction, some people in gym classes are not going to listen to what I'm saying, and are going to be reckless with their bodies. This happens less in studio classes, and I'm sure there are reasons for the difference, but it's not important right here. When I teach at the gym, I limit risky poses (like shoulder stand) to limit risk in general.
-Most of the class is there strictly for the purpose of physical exercise, and is more interested in ab work and hamstring stretches than they are in inversions or in poses with less immediately graspable benefits. I teach more esoteric poses anyway, because I think it's important to represent the full practice, but I tend to focus more on the basics.
-The room I'm teaching in isn't set up for yoga -- the walls are either brick or mirror, so poses at the wall don't work.
-More complex poses benefit from more hands-on adjustments, which I'm not as comfortable giving in a gym environment as I would be in a studio environment where adjustments are expected/the norm. And some gyms have policies prohibiting physical contact altogether.

That said, I think it's possible to teach a rich, varied class within the context of a gym, and I do my best to do so. But there are challenges in a gym that one might not face in a studio setting, yes.