Also known as, "the worst night of Teddy Long's life." Even worse than the time The Undertaker abducted him, because Long can forgive that. But no tag team matches at WrestleMania, playa? You're just crossing the line, now.
WrestleMania 3 had no Tag Team title match, but there were five different types of tag match on the show in general. WrestleMania 12's lone traditional tag match was on the pre-show, but did have a six-man on the main card. The card for WrestleMania 21 included seven singles matches, a six-way ladder match, and a pre-show battle royal, but certainly no tag team matches.
Every WrestleMania since then has had some type of tag team match, whether traditional, handicap, six-man, eight-man, or even twelve-man. Thus, WrestleMania 21 is the odd-Mania out.