I know this has been brought up before, because as it stands, event/seasonal cards do not count towards the complete set ex cards.
Problem is, when you look at a super pretty seasonal bikini clad gal, you end up recycling the non seasonal version. No point in having 6 copies of a card when you can only use 3.
Other reason, I don't think anybody owns a single one of the ex cards (at least with the current playerbase, I think HB and Rex had a set)
So yeah, that summarizes it. No point in having cards if no one has them.
P.S. Hand Turkey Contest Ex card made with lurrrrrves?