I think we may be operating from a false assumption in saying we haven't seen that. The UK variant was identified from a sample taken in Sept. It is very, very likely that it has been spreading here more or less undetected since not long after that, since US-to-UK travel was possible with limited restrictions for several months after the variant emerged. We just didn't know it until the variant made headlines in the UK, at which point we started looking for it here (and have subsequently found it to have arrived independently in a number of states). There's also mounting evidence that the LA area is dealing with a different, but also apparently more contagious, variant.
It seems possible, probable even, that we have been seeing the consequences of new strains. We just didn't label our fall surge as such because of a combination of a lack of the genetic sequencing that allows for variant tracking and a cultural bias to look for explanations in individual behavior rather than systemic shifts to explain changing data patterns.