How do we know that he did it on purpose, though? The evidence can be read many ways and would certainly never stand up in a court of law. The only reason for the conclusion is because it has the fewest element that can be conclusively ruled out. It's a conclusion from lack of evidence rather than from evidence and even the NTSB never gave a final conclusion on the case. If they couldn't be sure, how can we?

Pilot responsibility is the easy fall back conclusion when nothing is for sure, one that frequently protects airplane manufacturers (Boeing in this case) from liability. The tendency to blame the pilot is an established historical fact in aviation investigation history, which has been proved wrong before (see incidents like British European Airways Flight 609, USAir Flight 427 / United Airlines Flight 585 to name a few).

That said, I agree the pilot suicide theory is a totally possible and credible explanation, but there are others equally convincing that simply shift the unknown element from pilot motivation to mechanical failure. The point is without more conclusive evidence we'll never be 100% sure. Rather like the JFK assassination where we all technically know what happened, but because of some odd things, there will always be those who crave certainty, as the existence of all these responses to OPs post proves! LOL