How did Walberg not get nominated?

I watched this last night and I thought Walberg's work was at the very least worthy of a nomination. He was great in those classroom scenes and you could tell he put a lot of work into the role (not to mention sacrificing his physique that typically lands him other roles, although that doesn't justify a nomination).

Perhaps there's something I'm missing. One could argue that he was just the usual Mark Walberg but I see it the same way with DiCaprio and Cruise. They always do a great job but you pretty much always get DiCaprio and Cruise save for an accent here and there. Yet people tend to lobby for them to get nominated. Either way, I felt like he deserved a nod.