I don't understand "why did X happen?" presupposes X happened. We seem to be at the level of X pretty obviously did not happen but people believe it did.

Ah, I see what you mean. I my personal experience, those that believe in “alternative facts” typically believe in different narratives around the same thing and confuse the narrative with the fact.

For things that did not happen? Yeah. I am not sure there is something that can be done beyond pointing out inconsistencies in their reasoning and proves. However, typically, those things are about believes that mascaras as rational reasoning, and there is nothing you can do about beliefs.

Remember, after WW2 there were people in Germany who did not believe the Allies that Hitler and Co did terrible things.