Many languages now compile down to Java bytecode, so they can run on any JVM. What better way to draft behind the hard work the Java team does to ensure compatibility and ensure your code can run on a variety of platforms? Clojure, Scala, JRuby, Rhino -- the list goes on and on.
This can be an advantage for integrating, say, Ruby with Java, but it inevitably turns into a hassle. Suddenly it's not enough to make sure the Java code compiles with the libraries you've got. Now you have to worry about N languages running peacefully in the same JVM. When it works, it's a testament to the power of the Java stack, but it can be a headache generator for everyone.