Writer/director Christopher McQuarrie recently told the Light the Fuse podcast that the decision to go with two films came out of the need to have the space to tell a compelling emotional story, not simply for lead character Ethan Hunt, but the rest of the IMF team as well.
Not only that, but the two movies were going to be filmed back-to-back and would be set to release only about one year apart from each other.
While this is certainly not the first time a franchise has made this move, it was an intriguing move from one that had already made six films without going that route before.