Why, even though the boys elect Ralph as their leader, do you think they leave to join Jacks group?

At their age, they are much more interested in "fun," and the novel is clear when it points out that as they descend into savage behavior, more and more of them go over to Jack's side.  On top of everything else, they realize it is safer to be on Jack's side.  He has become a kind of leader who can talk them into almost anything.