Many researchers say that, for the Mexica, political power as well as religious belief is likely key to understanding the scale of the practice. Theirs was a relatively young empire; during their 200-year reign, they conquered territory all over central and southern Mexico, sometimes facing tremendous resistance from local communities (some of which would later ally with the Spanish against the empire). Spanish chronicles describe Tenochtitlan's sacrificial victims as captives brought back from wars, such as those fought with their archenemy, the nearby republic of Tlaxcala. Subject peoples in the Mexica Empire were also sometimes required to send individuals as tribute. "The killing of captives, even in a ritual context, is a strong political statement," Verano says. "It's a way to demonstrate power and political influence—and, some people have said, it's a way to control your own population."
https://www.sciencemag.org/news/2018/06/feeding-gods-hundreds-skulls-reveal-massive-scale-human-sacrifice-aztec-capital 
