coz native americans had a population explosion never seen in human history
imagine a group of 3000 people from North Asia, found 2 entirely new, huge continents with no people on them. their population exploded....
sexual selection is the result of evolutionary pressure. in such new scenario, there was much less pressure on sexual selection.
you're ugly af? no problem, just find another ugly af foid and breed 10 kids. coz there're so much new land, so much new animals to hunt, even an idiot would survive and thrive.
and that 10 kids turn into 100 grandsons. 100 grandsons into 1000 grandgrandsons... you do the math
same thing happend in lesser scale with other people, such as Slavs (they filled the vaccum in East Europe after the Germanics got genocided by the Huns), the negros (the Bantu people) whose population exploded after they adopted agriculture and they replaced the native bushmen population