If you are talking about aztecs it is because they believed the sun gods need human blood to fuel them
Because they wanted slaves
I tried to get help Boone helped on this question for me
This growth was rooted in the sustainable management of the region's valuable resources found throughout the ridge lands. For more than three millennia, the Maya were able to support and maintain their society's growth by forging an unusual alliance with their environment. This alliance was a balancing act that supported the development of the Maya civilization over some 40,000 sq km or 15,440 sq miles of space and across some 15 centuries of time.
