IHMO, if you just look at it like any other form of Tribalism (i.e. Political, National and even Sports) it absolutelly makes sense that people whose identity is a Religion (or a Political Party, Nation or even Sports club) cannot emotionally accept that those of "their tribe" are bad since one of the core foundations of Tribalism is exactly that "we" are good and it's the "others" that are bad.

The massive and for them extremelly important mental and emotional construct of self-validation and self-aggrandizing such people have based on their identity as member of a tribe (all those "we members-of-tribe are something-good" beliefs) - often at the cost of personal achievement recognition or as compensation for lack of personal achievement - would collapse if they accepted that the basic axiom of "membership of the tribe makes/proves people good/superior" is not true, so at the most basic psychological level they're driven to not question that axiom and to deny anything that might disprove it.