Java's big thing was "same code, run everywhere". Even before Android, apps and games on my 2002 flip-phone were written in Java Micro Edition.
1. Write Once, Run Anywhere
2. GUI in the standard library
3. Way less complex than hyper-complex C++
4. Garbage collection
5. Memory safety
6. Applets that ran in the web browser
7. Comprehensive standard library
8. Free compiler and JVM for all major (at the time) operating systems
Some of these things have ultimately not worked out, but at the time, all of the aforementioned features helped propel Java to huge success.
the fact you could get roughly the same order of magnitude performance as C in an environment that didn't need tons if #ifdefs to work on various platforms was amazing.