The sweat lodge ceremony has long been an important purification rite for Native Americans and remains so today. No one really knows for certain why Cahokia declined. Around 1350 its population began to decline and by 1400, it was largely abandoned. No evidence of a major war has surfaced. Speculation runs the gamut from disease to resource depletion.
History is replete with irony.
By the time Europeans arrived in the area, there wasn't much more to see of Cahokia than that which first greets today's visitor. By then, Natives had dispersed into the various tribes of the Mississippi River basin and Midwest. They were thus easier for Europeans to subdue. Had the same Europeans arrived 200 years earlier they would have likely ended up as a ceremonial sacrifice. When Cahokia was at its height, Europe was a technological and cultural inferior.