It requires developer and testing time, plus hardware to test with, and this project isn't exactly one of the more popular ones, so developer count and time is limited. I'm loosely looking into making it easier to implement very basic support of other targets (along the lines of "can boot dR, run UARTs and ESCs, but if it doesn't work a 100%, tough luck"), but I've been lazy lately (FYI I contribute only here and there), plus I'm only willing to buy so much hardware I don't really need (out of my own pocket).
Plus, if target support is too wide, the audience might grow too large, which involves more user support than the devs have time for.
There's a lot of OP Revolution based FCs, e.g. the Omnibus F4 stuff, and we cover a lot of it. We should probably put a list of those known working on the wiki.