- Sun already had a massive presence as a big-iron enterprise IT company, and machines running Solaris were everywhere; when such a primary vendor starts pushing a brand new platform, many of their customers follow
- The language itself is reasonably well designed, if a little verbose, but very consistent and well documented
- The Internet was booming and the prospect of writing write-once-run-anywhere applets for the browser JVM with superior interactivity compared to anything else at the time was very compelling
It was a win for everyone. Big enterprises got to ditch cobol, it generates oodles of services revenue, you need lots of boxes and it drives sales of RDBMS and other high margin stuff.
Huge, enormous ecosystem - and plenty of developers ranging from super-junior to rockstar. It's somewhat enterprise-driven ie: it never breaks backwards compatibility