JVM Languages

What's the deal with so many JVM based languages nowadays like Clojure, Scala, Ceylon, Groovy? Besides obvious multiplatform support is there a reason for them to be JVM based or even make JVM ports of current languages? I still don't get the point of stuff like Jython.
Let's see if we can get at least a few posts without mention of FORCED COLLECTION OF GARBAGE

Ubiquity of the JVM mostly. The CLR is a better alternative in my opinion, but people are scared MS will ruin their day if they use Mono.