Maturity and longevity, mostly. For really serious projects, I prefer to mitigate my risk by picking something that's been mainstream for a long time and isn't likely to die off. I'm not saying Clojure, Scala, JRuby, etc. are going to die off. Just that it's too early to tell.

Also, it's not exactly the same ecosystem. Yes, you can use all the Java libraries. But the ecosystem also includes the dev environment/build system, the written material, and the peopleware associated with the language. You don't automatically import all of these from Java just because you're using the JVM.