Mostly due to the security risk. Especially on Windows. Even being a software developer (read: not your average computer user) and taking extra precautions (like click-to-run for java use in browsers) I've still gotten hit by malware that takes advantage of security holes in the Java runtime.
I would think that, as a Science Fiction writer, separating yourself from current technology as a whole would allow you to be more creative in how you perceive the world(s) you are creating.
Why hasn't MSFT hired this entire team yet? They want the .Net platform to be pushed as far out as possible and Mono is a great way to do that. Plus it'll give MSFT +1 Open Source.