Why is there so much misdiagnosis?

One reason is that bipolar disorder is easily misdiagnosed as depression. This makes sense, as when people are depressed they will often go to their primary care doctor looking for an antidepressant medication ( 80% of the antidepressants are prescribed by non-psychistrists). 

During the visit, symptoms of mania, hypomania, or mixed states are easily overlooked by the doctor or not mentioned at all by the patient. For many with bipolar disorder, unless the mania or hypomania is so severe that family or friends intervene and bring them to a crisis center or emergency room, they may not not remember these episodes, or perhaps recall them as positive experiences or nothing out of the ordinary-- " I just had so much energy, I could go for days with very little sleep; it was wonderful!"
Other common misdiagnoses include schizoaffective disorder and schizophrenia--
This also makes sense, because if someone presents with delusions, hallucinations, and/or mania, their condition can be indistinguishable from psychotic disorders, such as schizophrenia.

Source:Bipolar Disorder-AnswerBook,Charles Atkins, M.D