Just spitballing but maybe at that altitude humans tipped some sort of scale where they were more efficient predators of whatever was up there but below that al

Just spitballing but maybe at that altitude humans tipped some sort of scale where they were more efficient predators of whatever was up there but below that altitude the larger pleistocene megafauna of the time represented a higher predation risk to humans themselves or there was less post kill (scavenger) competition.