Add to the fact that San Fran has a mediocre to subpar defense in nearly every category and the fact that tackles are not even acknowledged as a real stat by 90% of the NFL's teams, there's absolutely no reason Willis should even come close smelling that award.
This article is a good read:
49ers have one of the best run D's in the league, and Willis is the biggest reason why. If you'd watch a Niners game, Willis would be the first person you notice. He's everywhere. Constantly stopping the runner at or behind the LOS, constantly has good coverage, he's ALWAYS around the ball. He literally has no flaw in his game. If he was allowed to rush the passer more often, he'd easily be a double digit sack guy too.