The primary reason no doubt is because it enables him to deliver over the web. There are other languages for this but I have no doubt that in Notch's case I have no doubt its because he feels he can be productive in it and that is because it is similar to what he knows. He was asked on reddit why he chose java and he had a really simple answer:

"Because I know it"

Given the massive language debates that are out there I thought this was pretty refreshing and indicative of a successful person whose focus is elsewhere.

Myself I think Dart is a massive improvement over java and I could probably debate it all day. But then again I'm not as successful as Notch, maybe I should ease up on the debating.