I suspect that a major factor is that when people feel that the questioners are really not concerned for the truth, but are just interested in using the issue as a way of attacking, the tendency is to close up.

It's a universal way for people who have something important in common to respond in this kind of situation. That's not saying it's the right way. In fact it's likely to be the reverse of the right way - there is a lot to be said for refusing to treat an attacker as an attacker even when the attack isn't veiled. And lot of the time what feels like an attack isn't in fact intended that way, and responding to it as if it was is very damaging.

I think something analogous to that, a kind of siege mentality, is a main factor in bringing about the cover-ups and so forth. That and a reluctance in many cases to believe such accusations can be true. (Which in some cases they will not have been, which complicates matters still further.)