Ignorance. They know nothing about it and that allows people to fill the gaps in their knowledge with whatever bias they have. It's also true that all people hear about are Islamic terrorists. Obviously the jihadists represent a very small number of Muslims.
Very true, just because these extremists are Muslims doesn't mean that all Muslims are terrorists. In America, the religion of most offenders are Catholics, do we hate Catholics? Are we saying all Catholics are criminal? No. We should broaden our thinking. We shouldn't hate the religion the extremists represent. We should hate terrorism altogether.