The JVM is really mature, but its primary language is one many people hate, so people would complain about choice of platform even if they had gone that way. You may as well ask why Ruby on Rails wasn't Java on Rails. The answer is that 37signals didn't like Java.
And V8 and JavaScript already existed when Node was created, so they were building in an existing ecosystem. Again, JavaScript was already one of the dominant languages on the Web. That's a big argument in its favor.
To my understanding, JavaScript had one more advantage that put it over the top: what didn't exist in its ecosystem — namely, a culture of synchronous development. This meant Node wasn't fighting against the idiomatic way to do things. JavaScript was already highly async, and since this was the whole idea behind Node, it made JavaScript a less-bad fit than most other languages. Other languages were considered, but the fact that libraries for those languages were largely synchronous was viewed as a downside.