Word is he never even got hit in training. Just wasn't cut out for it. Bit like someone with vertigo applying for a job as a roofer.
Brock was more likely traumatized from the Carwin fight. That was probably his first proper beating, he's ever received. A fighter without fear, has limitless confidence in his abilities. Brock's just isn't mentally strong enough, period.
All he had was size, he didnt have enough time to properly develop his skills, and he probably didnt need the money or have the drive. Him retiring immediately after the Overeem fight shows his heart isnt in it.
Thats another part, despite him winning it was tough and probably the first time he ever had that happen to him, as going by rumour he never got hit in the face in sparring. Look at Haye, despite winning, as he moved up in weight, and with fight you could tell his eye of tiger was gone, i remember watching the Ruiz fight, and he had smash Ruiz around for 9 rounds yet Haye looked busted up...I dont think his body or mind really wants to go through that type of hard physical battle hence why he throws so little, and the fight with Wlad panned out that way. It will be the same with Vitali if he dosent fight. Also one has to understand the mental aspects too, lets looks at Chisora, he came in 20 pounds overweight and at press conferences looked shy and timid with T. Fury...looked like he wasnt there, then with Helenius he is in shape and back to his winding up opponents mantra...so sometimes we have to look at the specific situation. Perhaps things behind the scenes are affecting the fighter.