It has been acquired for 2 billion dollars, it has had 8 years in which to clean up and optimize that code. Either the all the people working in it are very incompetent or it's actually Java to blame :^)
2 billion dollars and 8 years hasn't even gone into such an effort though. The complete rewrite which serves as the codebase for the xbox, iOS, etc.. version is where the majority of that time, money, and effort has been spent. The mobile and console markets were apparently seen as as a much more lucrative use of time and effort vs optimizing the legacy desktop-only Java version which the author himself agrees was poorly written.