I can’t explain to you why this wasn’t flagged. But I suspect the refs noticed Michael Crabtree’s chinstrap wasn’t buckled BEFORE he got hit in the helmet. Of course the helmet came off, it was never properly on.
Crabtree is exceptionally stupid, and goes out of his way from time to time to prove it. From the lunch with Deion Sanders, to the rookie holdout, he’s a disaster just as often as he’s good. He’s better than Richard Sherman gives him credit for, but maybe Sherman just meant Crabtree was “mediocre” in a mental way.