Why do people always talk about Islamic terrorism and blindly ignore numerous acts of Christian terrorism?

It's called confirmation bias. The mind automatically emphasizes and prioritizes information which confirms already-held beliefs, and is less likely to value evidence which suggests to the contrary. People are stupid is the short answer. And while you're at it, don't forget the IRA, which was funded by donations from american benefactors, as well as the Ulster Unionists. Between them they've killed hundreds of people. And what about the Crusades?