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 agree with you. People misperceive Islams to be bad peoples, because they are biased by what they see in the media, they only believe what the media says and don't listen to anyone else. I believe they need to do a thorough research before they can judge Islams for who they are.