One of the problems of "reductionism" — explaining a complex phenomenon with a single cause or element — is that it oversimplifies complicated issues for the sake of a convenient explanation.

It's practical to look at the brain and its parts to explain mental illnesses. But does an easy fit with scientific attitudes and technology make it the right approach?

That's a question of authority. Science isn't (officially) allowed to make that judgment.

When it turns out that the brain-stuff doesn't match up to the mental-stuff, the whole thing will blow up.