because he's a monstrous athlete and a really skilled boxer. also because benavidez clearly wants none of it.
i see benavidez as a really tough guy with great volume and more than decent power. he is always the toughest guy in there, and often wins by having his opponents punch themselves out on him before he overwhelms them. he's got to be a nightmare to fight.
that said, i don't think that really works when you're faced with someone who for once holds all the physical advantages. morrell is more explosive, powerful, has a reach advantage, he's about the same height, and as far as i'm concerned he's a far better boxer.
unless morrell gasses (and that's not impossible, especially with benavidez), i think he dominates.