I posit several reasons:
1) Pretty much every school lets out for a four-day weekend, five-day if you are in a region that celebrates the first day of buck as a holiday.*
2) There is no reason to get up early unless you are involved in food preparation. Even then, many families don't eat until late afternoon.
3) Even if you somehow are hung over, you're going to spend most of the day asleep in front of the television anyway, so no one will be the wiser.
4) There really isn't any religious or hard-set traditions that prevent you from letting loose. Sort of like how you probably don't get rip-roaring drunk on Christmas. This is a tenuous assumption at best.
Anyway, those are the best reasons I can think of.
*This is not a joke.