I don't know. However, here's some hypotheses, some combination of which may be most of the reason: Even if depression and suicide attempts aren't more common

I don't know. However, here's some hypotheses, some combination of which may be most of the reason: Even if depression and suicide attempts aren't more common, very smart people may be enough more competent that they succeed more often. Most attempts at suicide don't actually result in death. It could be that the easy wins in building a smarter brain happen to lead to a genetic predisposition to depression. It could be that smarter people are more easily able to internalize that there's no ultimate point to life, and less able to just have faith.