When they fought in 2013, the respective win percentage of their opponents was 86% (Mayweather) to 79% (Alvarez). Mayweather had, by this point, beaten Genaro Hernandez, Diego Corrales, Jose Luis Castillo, Arturo Gatti, Zab Judah, De La Hoya, Shane Mosley, Ricky Hatton, Juan Manuel Marquez, and Miguel Cotto, just to pick some of the bigger names. Alvarez had beaten Kermit Cintron, Shane Mosley, Josesito Lopez, and Austin Trout, all of these wins (the biggest names on his resume) coming right before he fought Mayweather. Mayweather had won his first belt in 1998, seven years before Alvarez turned pro in 2005. When they fought, Alvarez had been a pro for eight years while Mayweather had been pro for 17 years. Alvarez had been crushing cans in Mexico for the most part until 2010 and 17 of the guys included in those 44 fights had losing records. Of the guys Mayweather faced only 8 had losing records. There was a big gap in terms of both amateur background, professional experience, and the level of competition the two men had before they fought.
Also, I'm pretty sure Alvarez is on PEDs, so I'm not arguing against that.