SB MVP, much like the Heisman are pretty much QB awards. That is unless a player single handedly wins the game for their team. Since Mahomes led the 4th quarter come from behind victory, Williams was going to have to have a SB record game. Had they won and Mahomes played like he did in the other 3 quarters Williams probably would have won.
The NFL tends to push the league in the direction they want. Mahomes being the perceived face of the league, left him as the only choice for SB MVP.
I would have been ok either way, hell I would have been ok will Sammy winning it as well.