Consider this: If Fred had stayed in after SC, who did he help? None other than McCain. The best endorsement/help he could have given McCain would have been to remain in the race.
That was my thinking, actually. I would have liked Fred to stay in the race to try to push a brokered convention. If Huckabee were to drop out soon, Fred might have had a real chance of winning. But Fred's staying in would have made a McCain victory in winner-take-all Florida much more likely, and that would have been dangerous.