It seems like there are other bits of evidence of pre-clovis human habitation, but taken individually they could be dismissed as due to some sort of misidentification, contamination of the site, etc. One the other hand this site seems to have very extensive evidence of long term habitation, backed up by several independent but mutually consistent dating methods. Add back in all those 'one off' finds as well and the clovis first theory starts to really break down.