Rubinius has been traditionally lagging behind in features (e.g. for a long time didn't implement 1.9 string encoding properly) and wasn't necessarily _that_ much faster.

Also, people that do want to work at the core of a language usually have knowledge of multiple and don't mind using C or any other implementation language.

Ruby stdlib is mostly written in Ruby, btw.

Still, with Rubinius 3.0 coming up, I'm very interested what is coming there.