When starting web development, I found it easier to learn Ruby and the Rails Framework from scratch than to use Java which I had several formal classes in. I think that says something about Java's documentation/community support.

Ruby/Rails has railstutorial.org, tryruby.org, rails for zombies/code school, and railscasts. All these high quality beginner resources made using Ruby instead of Java an easy choice.

The other reason for me is that I find the code I write in Java to be too wordy and found the more concise syntax of Ruby refreshing and fun.