It's the same problem as with other small OS projects: not enough hardware support to run natively unless you happen to have the same hardware as the developers. I looked into it, but would have to run it hosted on Linux or FreeBSD, so that doesn't buy me much.
Maybe if they ever build a version for common ARM SBC like rpi or beaglebone, that would be cheap and easy for people to run it. I think you can run RiscOS on rpi, actually.