Media Bistro has a list of reasons people won't follow you back on twitter.
I think it's a pretty good list.
The only thing I would add is that if you protect your tweets (ie you require someone to ask if they can follow you) you won't get followed a lot.
I never request permission to follow someone, and I block people who do from following me. The whole point of twitter is to expand your network, and protecting your updates doesn't do that.
(yes, this post was up at QueryShark this morning. For some reason, I didn't click the correct blog. Argh! Morning!)