Well, they can't exactly use the Russians, what with them being allies of the cheeto in chief.
And they can't go back to using black people as the fearmonger target yet, because they gotta break things down a bit more.
And, since the gays still have too much support to totally sell that idea to the less crazy Republicans, they can't go after them.
Gotta have the specter though, and those trans people are the perfect target. Small numbers, already dealing with attacks, limited by medical needs. Perfect target for the playbook.
There's more than one reason trans rights being human rights matters. The big one is the fact of it, but the attacks against trans people are the current battlefield for a fight that's been going on for at least since Nixon, in an obvious way. Longer if you divorce the struggle for civil rights from the southern strategy and look at it in context, because the line between the fight against slavery (not just in the US) and the fight for trans rights is fairly direct.
Trans rights are human rights. We can't forget that, ever.