[USAnum1]

I'd hypothesize a long history of relatively high population density created the cremation culture. Humans have been so successful because we adapt. Although the original motivation may be murky, I'd bet many arbitrary cultural institutions are actually adaptations.

[plonh]

This seems clear when you compare ancient religious texts side by side with what was known about the natural world in their respective times. Unclean animal species, menstruation rituals, etc.