New evidence actually shows that it wasn't. Recent archeological evidence at Cahokia shows that after it's period of depopulation due to flooding and droughts, it's people returned.

It later suffered depopulation after European contact however of course, for the same reasons as other tribes (disease, warfare, etc.)

https://www.cambridge.org/core/journals/american-antiquity/article/abs/after-cahokia-indigenous-repopulation-and-depopulation-of-the-horseshoe-lake-watershed-ad-14001900/C678D62BE9FAB07C9283D62BE9757685