I would suspect that one of the biggest problems is that until recently alot of the medical establishment, including psychiatrists used to talk about the "gene" that controled schizophrenia or the "gene" that controlled bi-polar disorder. Nowadays I heard that just in talking about autism we are no longer talking about one gene but in the interplay of perhaps hundreds of genes, and the same might be true of ADHD and Bi-polar as well.

Complicating this is that not all the same medication works with all patients with Bipolar disorder or schizophrenia, which suggests that while the symptoms they suffer from are similar (I would argue about this also) it would seem that the source of the symtoms come from different causes. In short, you have a name not of a disease, but of a list of symptoms, which is common when dealling with mental illness.

Added to this is th growing evidence that it is a blend of environment and genetic factors that lead to the emergence of these problems in most people who have them, and the old standard deterministic medical model is severely challenged, in some ways by a new form of itself. Some of the older doctors have a hard time with it.