Some major terrorist attacks were committed by Muslims, it's just a fact we have to deal with. No doubt they were crazy and acting outside the commands of the Quran but they were still Muslims. I do believe everyone knows that not all are violent like this though, the vast majority of Muslims are peaceful and add a great deal to society.
I guess mostly because the Muslims don't stand against the extremists in their religion. There are Christian extremists who are not tolerated and are punished for extreme acts against others. Muslims do not reign in their own. Therefore many assume the whole religion tolerates terrorism. Plus the religion itself was based on the murder of the infidels which means anyone who does not believe as they do. Whereas Christian based religion teaches love one another as God has loved you. Huge difference which creates divisiveness.