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.
I don't consider terrorists as a part of any religion. Religion teaches to love and not to kill.
Which religion is that? Christianity certainly don't, and Islam seems to mirror it. Judaism has always been pretty open about being exclusionary in WHO they love.
Now, they do all talk out both sides of their mouth... "Love everyone as your neighbor, but stone gays and rebellious teenagers" etc, etc, ad nauseam