Ultimately it comes down to, some drag queens decided to do this and they are. Trans people haven't, at least not to this level of visibility. Maybe some trans people participate in drag reading hour. I dunno, I've never really engaged with it.

But that pretty much applies to any time the question is "why is group a doing something instead of group b?" It's because group a decided to do it and group b didn't, or because they both did but group a was more successful/visible than group b.

Edit: also, while the answer might seem obvious when put that way, I don't think it's a stupid question. IMO it's better to ask a question if you're not sure. If the answer is obvious, then you'll know one more obvious thing than you did previously. A desire to learn and grow isn't stupid.