For the same reason that alternative implementations of Python never got the kind of traction they deserve, because 90% of the gems, libraries and everything ar

For the same reason that alternative implementations of Python never got the kind of traction they deserve, because 90% of the gems, libraries and everything are specifically tailored to MRI and it's always harder to play catch up. Also, the strong anti-Java culture in the Ruby community probably didn't help (even though JRuby has nothing to do with Java, or very little).