Board Thread:Legitimate Canon Questions/@comment-24821146-20150723133321/@comment-7603754-20150723141029

He was vulnerable because of what they did to save their parents. It couldn't possess Scott because he's a werewolf and you can't be two things at the same time. There's no on show reason for why the nogitsune chose Stiles over Allison. Derek expressed disbelief when someone brought up the possibility that Stiles could be it. But for anyone to answer your why was stiles chosen question, it would have to be done on the other board.