Java hopped onto the Object Oriented Programming bandwagon just as it was getting a lot of buzz. To a lot of people, Java and C++ are the canonical Object Oriented languages. (Sorry, Alan Kay.) Of those two, Java has the shallowest learning curve.
Java was also many developers' first introduction to a language runtime with a "fail fast" approach to fault handling. Java featured runtime bounds checking and runtime type checking. Couple that with the very detailed (for the time) stack traces that the runtime produced, along with automatic memory management, and you have a powerful set of tools for building (relatively!) bug-free software. (For comparison, imagine trying to debug a C++ program that intermittently crashes on an off-site customer's computer.)
Object Oriented design, garbage collected memory management, and runtime error checking all had roots in other languages (Smalltalk and Lisp come to mind) but Java was the first to package all those things together in one well-documented, industry-oriented tool.
Most of those predecessor languages were widely perceived as by academics for academics, which brings me to the final piece of the puzzle: Sun nailed developer outreach. It doesn't matter how good your language is if nobody has heard of it.