It wasn't just human sacrifice as a once in while ceremony. It was daily. And they sacrificed dozens even hundreds of people per day. One of those tribes, don't remember which one, believed that if their God did not receive enough blood every day, the sun would not rise the next morning.
I have to wonder what could have been if they did not kill all those hundreds of thousands of slaves. I mean to say, that at least part of the reason they disappeared so suddenly may have been due to the fact that they were 'sacrificing' a significant percentage of their population.
Jeez man, let them live and build and and farm and reproduce. A dead slave is no good to anyone. Apparently they did not see things this way.