[JBits]

At the time, Java ran on the web so he could easily share demos.

He used LWJGL, which probably covered a lot of his library needs.

Java has a JIT, so it's not comparable to Ruby and Perl.

[surround]

Minecraft gained popularity super fast in its early days, probably because it was a free-to-play game that worked in the browser (thanks to Java). Notch had no reputation or marketing at the time.