Everyone wants to think that they know more than the other guy. So they glom onto whatever idea sounds good to them. And sometimes they just want someone to blame for something bad, and the existing reasons given are emotionally unsatisfying; not enough "truthiness" for them. Once they've made up their minds that they know The Truth (because it feels like it must be true), that's it, it's set in stone and no mere fact is going to convince them otherwise. Rationalization takes over and everything they learn gets filtered through that belief.

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"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
Illuminati's Razor-The most complicatedly evil answer is usually the most correct answer. - Fazor
"Every book is a children's book if the kid can read." - Mitch Hedberg
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort