Agree with Case42... AskBot is pretty neat, but it's written in Python, which means it runs as an app and you have to restart the webserver every time you change something (if you don't do tweaking, that's not a problem, I guess). And it has dependencies, packages, etc. The install procedure is really messy.
Also being Python-based means it doesn't integrate as easily with Wordpress, which is also written in PHP. I was just more familiar with PHP and use Wordpress on my site, so Q2A is pretty natural for me.
Another thing is, Askbot didn't support plugins, if I remember correctly. I was hacking away at the core for a while, and Evgeny (the leader) was great about incorporating our changes, but in the end, it just seemed like more work than it was worth. Theming is also more complicated, I think. Q2A plugin and theme systems rock :)
A plus for Askbot, I guess, is it runs on Django, which is a pretty big project with some neat features if you are a developer.
Edit: Looking at the stats can tell you something:
602 questions, 431 users
2,316 questions, 2,171 users.
PHP => Ease of use => Widespread adoption => community involvement => quicker bug fixes and feature additions => Ease of use... and so on.