1) Java is, as far as I know, the most universally taught language in universities and even high schools/equivalent. It is used widely in the professional world and has a lot of codebase behind it to help ease developers into creating more quality apps, faster.
2) The overhead is not that great vs. native code, especially with well designed code that is JIT optimized and being monitored by an ever-more-well-designed-gc.
Really, I think these two reasons alone can justify why Java was chosen as the primary language to be supported on Android.
