I'm being told that thousands of people were sacrificed every year. I don't think anyone would particularly like to be slaughtered. If that many people were being killed against their will, why didn't they band together and revolt?
Not to discourage further answers (I would give a semi-detailed summary, but I'm not sure it'd meet the subreddit's high bar for standards and basically everything I would be saying is stuff the following links say and expand on better), but there's a number of great posts already on the subreddit that have to deal with Human sacrifice in Mesoamerican culture and role it had for the geopolitical scene in central mesoamerica/for the Aztecs and their political relationships and schemes:
Be sure to read not only the linked post, but subsequent comments in the comment chain and other top level comments as well for these, often the most relevant stuff to what you are asking, as well as the most interesting and informative stuff isn't necessarily in that main post.
[I vaguely recall it being considered a rule or soft guideline to tag people whose posts I reference here, so [/u/400-Rabbits, /u/snickeringshadow (who is the deleted account for some of these) and /u/Astrogator . Also, while he didn't do the primary post of any of these, /u/Ahhuatl gave a ton of great responses in a few of these posts.
There's some other good posts on this and related topics in the FAQ I didn't link here, here, and here as well.