He won because he put the bum down four times, AJ was exposed to the world as the hype job seasoned boxing fans knew him to be.
I said before the fight that it would be a big mistake for Joshua to try and force a knockout on Ruiz. Joshua despite achieving success so quickly is still a very green fighter with a lot to learn.
Joshua didn't even know where he was when fatso went on the attack. Kept asking his trainer what was happening to him? Did the bodybuilder mug Ruiz of his last whooper burger before the fight.