Different people are saying different things. The poll features a few I've heard and read. Please pick the primary reason. You might consider some of these coupled or related. All reasons factor somewhere, we know. Primary only.
He lost because Manny Pacquiao has carried his power up at 147. It's as simple as that for me. Cotto was winning the rounds until he was getting hurt.
He simply could not deal with Manny's speed and tenacity. His combination punching was excellent. Nor could Cotto hurt Manny. When those were established, it was only a matter of time.
Pac was able to take his bombs so UNBELIEVABLY. As a consequence, he was able to unleashed his own bombs knowing he can take cottos punches.
Manny's punches were coming at such awkward angles, punches coming at off beat timing and in bunches, that Cotto couldnt cover up well enough or brace himself for the incoming shots, even if the punches werent the strongest that he's ever received in his career.
He had the wrong game plan. Early on he was doing the right thing just jabbing and throwing body shots. But he got carried away and started brawling. As soon as Pac hurt him though in rounds 4 and 6 Cotto was just fighting to survive and that is the only reason he lasted as long as he did.
A combination of things but mainly due to the fact that Pacquiao was able to adjust and that Cotto never really had a plan B. It looked like he only planned at keeping Pacquiao at bay on the outside with his jab and welcoming Pac to fight on the inside thinking his power would be overwhelming.
You know what...... my 1st inclination was to say it was Pac's speed, because I think that that attribute was the single biggest weapon in the fight, but upon further review, I think it was Pac's power that did Cotto in. From all accounts, Cotto was training to deal with Pac's speed, but I don't think he expected his power to hurt him quite so much. And that surprise factor is why Cotto lost. He became undisciplined and thought he could out bang Manny and it cost him the fight. He was never the same after the 4th rd KD.