Mostly political reasons it seems. Java was always encumbered with patents and licensing restrictions especially for mobile usage, which came out the ugly way as we know with the latest Oracle's lawsuit. Even though Oracle didn't succeed, but nevertheless there was always a risk. I guess Microsoft didn't want to depend on that, but on the other hand wanted to leverage familiarity with Java to lure Java developers into using C#. Rather boring and uninspiring move (creating a close clone), but Sun can be partially blamed for not making Java truly free (like C++ for example).