Cam deserved MVP that year. Team went 15-1, crushed their way to the superbowl before running into 1 of the best defenses of the decade. You have to factor in his rushing stats and everything that meant for that team. I follow football pretty closley and I'd have trouble naming their top 3 WR's that year. And this was a superbowl team.
Oddly enough, only other guy that really warranted consideration for the award that year was probably Carson Palmer (if you stick to strictly QB)
https://www.pff.com/news/pro-2015-most-valuable-player
There is PFF's writeup of it at the time, they actually give it to Palmer