Two reasons. First, houses tend to act as chimneys--warm moist air rises, finds a way out near the top, and thus floors tend to be under negative pressure--outside air is being pulled in down low. Even if this isn't the case, most floors are sheathed with plywood which which has a high enough perm rating to function as an air/vapor barrier. I have seen a number of floors which were covered on the cold side with poly and there were no apparent problems, though it would probably be prudent to use housewrap on the cold side just in case.
I meant to add to this discussion that most foams cannot be exposed in a crawlspace. they must be covered with a thermal barrier like drywall.