I don't think this is applicable to python. Only so many developers can work on cpython without stepping on toes. This led to the decision not to have cpython to be an implementation standard, not an experimental language, while pypy led the effort for expanding with a different team of developers. This is a good decision, because there are still more than enough developers to maintain a stable, full-featured cpython that pypy can conform to (to varying degrees). There aren't any competing standard libraries, for instance, and it's made very clear which libraries work on which implementations.

Maybe if there were competing implementations to ruby, it would still be popular outside the rails community. But it seems as if the vast majority of ruby core developers work on the mainline implementation, or forks thereof.