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.
Don't know about ignorance. I've heard some damn good arguments to not like Islam that are based on some pretty solid facts, like straight out of the Koran. Having said that, I don't hate Islam. And yes, it's the fucking terrorists who did shit like crash planes into the WTC on 9/11, bombed it in '93, the Boston marathon bombing, and 7/7 in GB. I would say they definitely put a bad face on Islam.