I started out with textmate way back when. I actually started using NetBeans in hope that it would have decent support for Actionscript 3. I happened to find out about the Rails support later on & was very happy I did.
Here's some of the reasons I switched:
1) inline syntax checking
2) ability to click through to called method (command + click in os x)
3) ability to have multiple views of the same file (clone window)
4) ability to have split screen editing.
5) debugging
6) inline documentation instantly available for any built in, plugin or internal class (ctrl + space).
7) code completion / suggestion (this is not a lazy thing -- it's helpful for discovering what's available in any given class)
8) indenting in textmate was never fun. In NetBeans just command + shift + f and the whole file is properly indented.
Those are some of my reasons. Hopefully there's something else out there that can replace NetBeans for me. I've been extremely grateful for it -- especially at its price.
I would bet a lot of them started as Java developers (who generally must have an IDE) who were experimenting with Ruby and used the tool they had in hand. There are also probably a chunk who want a "real" debugger.
And it had RVM-like features before RVM became popular ... you could specify what version of Ruby and what server (mongrel, web brick or thin ... if you knew what you were doing) you wanted to run your app against
