My impression is that "behavioral" health was a term invented because anything that smacked of "insight" was thought to be unscientific by a certain crowd; they insisted that only external behavior could be verified scientifically, and that came to mean that only external behavior had meaning at all, scientifically. They felt, and many still do, that self-observation was too unreliable to be "scientific" -- and of course it does have many potential problems. So the baby got thrown out with the bath water, since some were too afraid of examining their own uncleanliness.

The term is used now as a sort of cover for all mental or emotional problems, since it has, for now, stuck, and people use it without understanding the original restrictions that were intended to outlaw the internal life.