Just the simple fact that there have been "first peoples" on the north american continent for nearly 35-40 thousand years. More and more evidence piles up every day that the "conventional wisdom" is willful disregard of much more than anecdotal evidence that the "first ones" got here from europe - not asia -
between the last two glaciation events, and came back to reestablish themselves at the end on the last glacial period.
By modern standards, NA north of latitude 40 would have only been uninhabitable between 15 and 27 KYa. Add to this recent evidence of "man" in Great Britan 700,000 years ago, compressing the habitation of north america to the last 20K years makes no sense at all.