Because it is slow. Compared to C/C++/langs without runtimes [albeit with JIT compilation, for long running applications this is becoming a non issue].

Compared to the current crop of dynamic language interpreters, waaaay more engineering time and talent has been poured into optimizing the jvm.

People I think forget that Java was a more user friendly C++; the price you paid was somewhat slower apps, but that's OK because you write more robust apps more easily. Rinse and repeat for Ruby/Python/Your Lang Here.