#2 Popularity
If you are Vince McMahon, you want your best guys and the most popular ones to headline Wrestlemania. WWE Championship is the most prestigious title in the whole of sports entertainment, and a Wrestlemania match for the same shouldn't be headlined by someone like Mojo Rawley.
New Day started off with a dead gimmick and slowly won the audiences with their insane charisma and in-ring work. They have consistently stayed on top of WWE's popularity ladder and their merchandise sells like hot pancakes.
It is hard to find a babyface superstar or stable who can remain fresh in the same gimmick for almost five years. Most superstars either get stale or run out of good content to keep their character interesting.
New Day have managed to do that with ease, and have captivated the audiences worldwide.