Everyone seems to have covered the "dumbed down language" perspective. And there's definitely truth to that. Enterprises hire tons of developers, but Enterprises are terrible at determining which candidates are good developers. So there are a lot of mediocre ones.

However, there is another side of the coin that must be acknowledged. When Java came out, there was no Python or Ruby. Java let you be enormously more productive than C++ for apps for which garbage collection was fine. The VM offered features which weren't common, like reflection. Of course, Java came with a huge library. Java was a great option for a JFDI (Just Fucking Do It) language, and there weren't a lot of other options back then.

This doesn't solve the whole mystery. Why didn't SmallTalk or some other options that were available at the time do better? I don't really know. Java just took off like a rocket, in the same way that pet rocks and hula hoops did. There were some good reasons, but those reasons can't explain the whole phenomenon. And now, after that adoption tsunami is complete, Java is the safe, established (but boring, mediocre) answer that Enterprises like.