I moved to Vim from TextMate during a period of my life wherein I was only developing on a 13" Macbook, and found that the amount of screen real estate required to run my terminal for testing, TextMate, and my browser to be unacceptable. With Vim I could simply run it in the terminal, split my screen with TMUX, and run my tests.

I've stayed with Vim because I find it has very efficient means of manipulating text, and tends to stay out of my way. I feel encumbered when using Sublime Text 2, or TextMate (though less so with the former).