Because he has more knockout power
Kelsey Mc Carson: “Pacquiao is a power puncher with unworldly speed. He devastates his opponents by hitting them both hard and often. In essence, he's a volume puncher who hits with force and rapidity, something that has made him one of the better offensive fighters in boxing history.”

Rafe Bartholemew:  “Pacquiao is such a dynamic and creative offensive fighter that when he decides to throw everything in his arsenal at Mayweather, something will work. Pacquiao will get the knockdown or two that he needs to edge Mayweather on the cards. Maybe, just maybe, he'll get a stoppage.”

Because he’s more aggressive
Bernardo Pillati: “It will be a tense, highly technical and caution-filled fight in the early rounds. Pacquiao will begin to loosen up from the fifth round, will show great explosiveness and aggressiveness, apply the best shots and dominate the battle for a clear unanimous decision.”

Samuel Ha: “Pacquiao is known for his relentless pressure and Mayweather is known for his passive stick and move style…Manny is never discouraged. There’s no doubt that Manny will be willing to throw 5 punches to land just 1 while Floyd will be content to throw 1 or 2 punches at a time (which by the way, can miss).” 

Because he’s more battle-tested
Nigel Collins: “I've never bought into the opinion that Mayweather would easily outbox Pacquiao. I thought it was a 50-50 fight when the match was first talked about years ago and I still think it's a 50-50 fight today. Whether they have declined with age or adjusted their styles in order to continue to compete successfully against younger men really doesn't matter.”

Carlos Narvaez: “We are facing the most difficult fight in the career of Mayweather. Pacquiao is presented as the toughest test for the pound-for-pound king because of his speed, counterattack and stamina. "PacMan" also has punching power and can be as slippery and elusive as Mayweather himself, whose energy we have already seen begin to wane in late rounds when pressed.” 

Because he has better offensive techniques
Samuel Ha: “The subtle feints and the crazy angles will pose a serious problem for Floyd, like it has done all of Pacquiao’s other opponents. The question is, how long will it take Mayweather to adjust to the feints and angles of Pacquiao?  Mayweather needs to be able to read them and not fall for them(easier said than done), or make Pacquiao miss because the more impatient he misses, the less he feints and the more wild he gets.”