I don't know that "all he had to do..." is accurate. Aside from some shennanigans, he did put his fucking heart out there. You're talking like it's not Manny fuckin Pacquaio on the other end of his boxing gloves, brah. Shit isn't that simple against him, and Manny's legacy is proof enough of that. One of the best to ever do it just beat a very motivated Bradley in what certainly appears to be his prime...and beat him soundly from my perspective.
From what I saw, Bradley had a plan and mostly executed it as best he could. Lots of heavy shots. Give Manny the power, and hopefully wear him down and overwhelm him with it over the course of a match. Lots of punches got blocked. Lots of punches were bigass wiffs. In the end, it felt like he ran out of gas. Not surprising considering he put his soul into damn near every punch.
Also, outside of that headbutt cut, somehow it didn't really look like Manny had been in much of a fight (face). Bradley's cheeks were swollen as shit, though.