It’s not hard to figure out. There are a lot of trans actors out there, but very limited roles for trans characters. When a cis gendered person takes a trans role, that only exacerbates the limited number of opportunities.
Add on to that the important perspective that someone who actually is that kind of person brings to such a role.
Like most things, in an ideal world where trans actors are given ample opportunities, it’s a non-issue. Same with actors of different orientations. But we don’t live in that world.