According to Deadline, a big part of the decision was to add 3-D to the mix, which is more popular internationally than domestically. I've been really surprised to see how much globe trotting Will Smith has been doing to promote 'MIB3'. So it will be interesting to see how the latest installation does without 3-D.
The only 3-D movie I've ever seen was last year's 'Conan: the Barbarian' remake. As you can see, the international box office numbers were actually better than the (terrible) domestic numbers. Imagine how much worse those numbers would look if they weren't charging you those extra dollars for 3-D. You can see that 'Avengers' is sneaking up on 'Avatar' in the 3-D gross department, passing Toy Story 3.
This likely also means next year will mean more Titanic 3D type re-releases.