Good question, almost all the refugees are Sunni Muslim's, ISIS is a Sunni Muslim terror group, so these people aren't threatened as much as if they were Christians or another groups. Christians are being targeted for extermination all over the middle east, if we're going to take in anyone, Christian refugees should be given top priority.
Since its a civil war, these people are fleeing because they don't want to get in the middle of it. If your a young man you may not want to be forcibly conscripted to join ISIS or to fight ISIS.
My question is, why aren't the wealthy sunny Arab countries all around Syria taking in refugees and why is so much pressure being put on the west to take them in?