I think part of it is a natural -- but unfortunate -- tendency to not want to admit how fallible the human brain is. No one wants to believe that, given the right (well, wrong) conditions, they themselves could do something that terrible to other people while in a suicide spiral. So we're reluctant to believe that anyone could do it.

Then we have the Malaysian government, not wanting to admit to the embarrassment that one of their most experienced, thought-to-be-solid pilots could do something like this, and that they all missed any possible signs that it could happen. As much as we're consciously skeptical of the Malaysian government's report and claims, I bet their words do have a subconscious effect, adding doubt to the murder-suicide theory.