because there is a lot of good academic and industrial work done on optimizing JVM in different universities and strong companies like BEA, IBM, Oracle.
3 years ago Microsoft published a small portion of .net framework on FreeBSD in hope to get momentum from academic society which never got a considerable attention from software  society.
lots of people predict that java  may be replaced by other languages  in future, but Java will stay as a platform which other languages (like groovy or ruby) may run  on it.