Because : http://www.ssa.gov/disabi... You have to have a history, documented. Not by a GP (general practitioner) you have to be diagnosed by a psycologist. It has to be something that can't be controled through medication. This has to be proven by trying different medications and having them fail to control the disorder. It has to be something that noticablly impacts daily life, in ways that can be documented. It has to be on the list.
I'm no expert, but it might be because bipolar disorder isn't classified as being a severe enough disorder for you to qualify. That sounds stupid, since it's obviously causing you not to be able to work, but for many people, it is a manageable disorder, so that might be one of the problems. Not really sure.