Desperation/Laziness, sometimes ignorance but tbh I think that's rarer than is hyped.
Basically it's a result of culture being so closely connected with race. One culture sees itself as being "invaded" or "chipped away at". New religions pouring into what was once a pretty much atheist place will leave people hateful of the "intellectual suicide".
But rather than do the sensible thing and attack against a specific culture / religion (that's politics btw), a lot of idiots screw both sides over by using racism.
Meh, worked for the Nazis.