Because Manuel Marquez is really, really good? - Pac is much more a “athlete” than boxer he had holes, think of a little Ali type…albeit more fundamental.
Marquez’s style works best when he has an aggressive opponent. He’s a counter puncher so the more his opponent throws, the more opportunities there is to counter. Pacquiao leaves himself completely open when he attacks. By completely open I mean head stays on the centerline, chin in the air, and he drops both hands. Marquez will make you pay for that. He could crack so Manny had to respect that. Any time Manny fought an elite counter puncher, he struggled because they all exploit that flaw in his game. Marquez exposed it but Morales was the first to fully capitalize on it.
Pac never had the defensive reflexes Ali did. He certainly had the hand speed but not the defensive reflexes.
Pac is a very aggressive whirlwind of a fighter while Marquez had the perfect style for him due to his counter punching mastery and also performed well against southpaws. A fighter with JMM's aggressive counterpunching style who exceled on making the other opponents lead always has the upper hand but Pac's explosive power, speed and athleticism helped him overcome that obstacle just enough to be able to fight on even terms with him. Pac had glaring fundamental flaws that gave JMM opportunities to exploit but it was never easy for him since he still had to fight tooth and nail in every fight. All it took was one big mistake for JMM to finally close the show with a nice counter right when Pac came in too over eager to get the stoppage. These two were just made for each other stylistically.
Two things: First, Marquez was an elite fighter in his actual prime. Second, Marquez was a counter-puncher well-suited to match Pacquiao's raw aggression (though Manny did seem to almost overwhelm Marquez through sheer speed the first round of their first fight). TLDR: Marquez is a fighter only slightly below Pacquaio's tier, but the stylistic advantage in his favor makes the fight an even match.
Marquez worked aggressively to pressure space and then counterpunch fighters who entered that space, he was a counter puncher, but it was more the case that he was a counter-puncher of the space - he wanted to trick you into the space and trap you into thinking that it was safe to punch. Pacquiao didn't need to be tricked. He wasn't stupid, but he was ultra-aggressive and sought opportunities to punch relentlessly. That's a nightmare for most fighters, but for Marquez it was welcome because Marquez spent time tricking people into behaving in this way. So this is a significant stylistic advantage. Pacquiao fights exactly how Marquez would want him to fight, and that's across the styles that Marquez embraced in his career - this fundamentally did not change despite the changes in Marquez. Of course, Pacquiao is still a huge puncher who is very fast, also nightmarish in its way, but that is the nub of it for me.
Marquez was an iron man, hard as nails, had a terrific chin, stamina, strength, heart, skills as a counter puncher and was Mexican tough .. that KO was brutal .. I'm still shocked Manny recovered as well as he did ..