Because the main theme wasn't catchy enough as is
We've said time and again how great the Super Mario 3D World soundtrack is. The team used the game's main theme quite extensively in promotional videos for the game, and it became obvious that they were taking the series in a more swing and jazz-oriented direction with the inclusion of a big band brass ensemble.
Well, Dj Cutman had other plans, and has infused the catchy main theme with some hip-hop flavor. I can certainly dig it, and would love to hear more of the game's amazing music get some remixing attention as well.
Anyone have a favorite track yet?