@crb I think a big factor is what spare devices developers have lying around. I imagine most people won't want to work on their main device, and so most development gets done on older phones, or weird and wacky ones bought expressly for the purpose of hacking on.
Still, I agree with you that it is a shame, before I got my Librem 5, I was looking to install on my Motorola, only to see it wasn't supported yet.