A lot of the wrestlers and writers are pissed at the US fans in a lot of states. Booing and chanting 'boring' during the WM main event is disrespectful. If you don't like what has been booked, don't watch. Don't go to the show.
When they wrestle internationally the fans boo the bad guys and cheer the good guys and it makes for a much better atmosphere. With the WWE now global, expect a lot of PPV's to start happening overseas. Plans already in place for 1 of the big 4 to happen in the UK.
Don't put on a match where one guy has limited his moveset to a half dozen moves, and crowd has no interest the other guy having the title as the main event of Wrestlemania.
That'll solve that problem.
It would have happened during that match regardless of if it was the main event or not. IF you gave AJ and Nakamura the main event, and gave them 40 minutes to build, you'd have a better reaction.
If you had the three minute Taker v. Cena squash as the main event, you'd have a better reaction (although there'd have still been confusion over why it was a squash).
Forced booking, and not reacting to the crowd has been a WWE problem for several years. The crowd reaction is going to be amplified at WM. Why expect differently?