A lot of it is that the JVM is used by devs to build large systems more commonly than .NET, and often work from that gets open sourced. So the JVM just has a larger presence in the developer community. Another reason is that historically many developers disliked Microsoft and SUN did a lot to foster goodwill among developers.

Oddly, SUN got purchased by one of the few companies everyone hates more than Microsoft. That paired with MS's recent shift to open source and opening the dotNet platform has led to softer view of Microsoft overall.