Maybe it's because you're one of the last 3 people still mining on Deepbit lol. Seriously though, I stuck with them for a couple years and when I switched to Slush's pool I started getting about 10% more BTC with the same speed at the same difficulty....
Once I hit next cash out i'm done with them. Was just waiting for someone like you to ask me why I'm still with them, when I have no idea myself.
Thanks for the feedback