That humans came to the new world multiple times before the native Americans arrived 10-13K years ago isn’t the BIG QUESTION.
The big question is what wiped them out?
The even bigger question is, is it still around?...
could be weather events. Consider the NC/SC storm surge. Flooding in Texas, tornados, volcanos from NM to the Pacific.
Or could be virus: European settlers to the Americas caught Hemorragic fever. The Indians also suffered from it; some populations may have built some level of herd immunity over time and some not. Carried by varieties of mice/rats drawn to subsistence farms/homes. Or, maybe some other virus/carrier. T-rex?
http://www.infectionlandscapes.org/2013/02/american-hemorrhagic-fevers.html
and,
“the epidemic of 1576 which killed 45% of the entire population of Mexico”