Or, more simply, he can't take a punch. He has no idea how to defend himself standing up, and he really doesn't like getting hit.
Well, in Brock's defense, he got pounded WAY harder by Shane Carwin. And somehow stayed in the fight, eventually winning.
But Brock didn't train right. He never learned how to take punches. His sparring partners needed to pound in training. But Bas Rutten said that Brock's trainers were not allowed to hit him in the face.
So, Brock was doomed against Cain.