It wasn’t climate change, asteroid impact, or habitat change. Human hunting was maybe a factor-
Why did the giant bison, mammoth, mastadon, giant ground sloth, etc. disappear so quickly? There are a number of hypotheses. This article describes some progress.
Dung Fungus Provides New Evidence in Mammoth Extinction. By Betsy Mason. Wired Magazine
If it was human induced, the evidence says it must have people before the relatively sophisticated Clovis culture.