Because you have not learned from history.
And so you are vulnerable to repeating history.
What happened to the Jews in Germany can happen anywhere to any minority given the right pressures on a population and a clever manipulator of the resulting rage.
Before WWII, people never believed that nations would ever drop bombs on civilian targets.
But very soon, they were doing so, right on up to dropping atomic bombs.
Never become so used to the thin veneer of civilization that you think it permanent--because it only works so long as a people are fat and happy.
But make them suffer, make them angry, and they will turn--and they will find some holy justification to murder their fellow man.