Removing the rose-tinted spectacles for just a moment, RISC OS is - quite frankly - a complete mess. Still hosted in CVS. Poor documentation. (Currently) closed source development tools (IIRC). Major parts of it still sitting in 32-bit ARM assembly. Scant hardware support. No virtual memory. No multi-core support. No 64-bit support.
Don't get me wrong. I'm every bit as nostalgic as the next Acorn person, believe you me. I'd like to see this spark fresh interest into what was (30 years ago) a really nice bit of kit. But there's a SHED load of work to do on this thing, if it's going to get any traction, and there's no immediate vision of where it's going. And given the state it's in, it's somewhat unattractive to developers who might otherwise find themselves interested in kernel/OS development in this day and age.