It's not quite so black-and-white. Most of our historical sources are people who definitely want to play up the human sacrifice aspect. The Europeans would do so in order to justify the genocide they would commit; the Aztecs themselves to demonstrate their legitimacy to power. So the extent to which Aztecs did human sacrifice is probably exaggerated, although most historians would agree that Aztecs were the most prolific practitioners we know of nonetheless.
A more important thing to consider is the actual context of sacrifice in Aztec religion. Their mythology held that the gods literally sacrificed themselves to bring the (current) world into existence, and they recreated these sacrifices both as a means of honoring their gods as well as ensuring the continuation of the effects of that sacrifice. This isn't actually all that far off from Christianity, which recreates the sacrifice of Jesus in the form of the Eucharist, which can (and historically was!) be seen as a form of human sacrifice. Indeed, in broader theological terms, Aztecs need to undergo personal sacrifice of their own flesh and blood, while Christians make do with merely sacrificing personal possessions or (more commonly) abstract desires.
The Aztecs had made so many enemies from slavery and sacrificing children in their rise to power, the Spanish had a broad choice of allies. Even if the Aztecs had instead sacrificed those 20k warriors on Spanish swords instead of the priest's knives, they still would have lost.
A more important thing to consider is the actual context of sacrifice in Aztec religion. Their mythology held that the gods literally sacrificed themselves to bring the (current) world into existence, and they recreated these sacrifices both as a means of honoring their gods as well as ensuring the continuation of the effects of that sacrifice. This isn't actually all that far off from Christianity, which recreates the sacrifice of Jesus in the form of the Eucharist, which can (and historically was!) be seen as a form of human sacrifice. Indeed, in broader theological terms, Aztecs need to undergo personal sacrifice of their own flesh and blood, while Christians make do with merely sacrificing personal possessions or (more commonly) abstract desires.