There's been a long-standing commitment from Sun to make Java backwards-compatible. It made introducing new features in Java painful (generics), and fixing bugs (java.sql.Timestamp vs. java.lang.Date) impossible. On the other hand, that huge Java Class Library really needs refactoring and compatibility breaking is the only way to improve the developer experience.
Python development goes on in a completely different manner. They are set on producing a partially incompatible version called "Python 3000" that will fix the major design flows of the language (e.g. crappy i18n).
I hope Java does the same or dies.