Because Jesus told his followers to “turn the other cheek”. Since Jesus said the right cheek, he meant specifically a slap with the back of the hand—or an insult.
So when Christians get insulted, we follow our teachings, not lashing out from an insult.
If the blacks had just "turned the other cheek" for their enemies there would still be Jim Crow today and the KKK would still be considered "respectable." Instead, they created a movement that was largely Christian and largely non-violent, but stood up and said, "Enough!" This changed both blacks and whites for the better (although there are still problems to be worked out and political correctness has made things more difficult).
I wonder what would have happened if Christians in Germany had early on and vigorously fought the cultural corruption that was Naziism. Instead, "turn the other cheek" became "turn your head and ignore what is happening."
I don't mean to flame you, but I do think that the concept of turning the other cheek has been over-used and misapplied.