Java was able to ride the wave of the web's rise in popularity, particularly with their marketing of applets and the "network as the computer", with the idea of downloading Java code as needed to supplement the software you're using at the time. That and there was lots of talk about how the JVM was going to be the runtime of the future for all languages.

It's funny how Javascript and the web have turned out to be a more popular target. Nobody back in the late 90s/early 2000s would have thought that. JS was a toy and html was going away in favor of Flash or Java applets or .NET.

At any rate, timing is a really important part of gaining widespread adoption. Smalltalk was way ahead of its time in the 70s, but too early to really take advantage of networking, as their target was stand-alone personal computers.