Because its still winter and herd immunity from natural infections is a gradual process not a steep cliff? At the very least it seems like those factors could be accounted for.
There have been several new variants that have popped up lately, many able to spread rapidly, and some showing the same mutations but developing them independently.
With the rapid spread of some of the variants, it would be foolish to assume that what we are dealing with now is the same thing as what we are dealing with before, and we really don't know enough about the new variants. Maybe they are spreading, but not making people sick enough, increasing herd immunity.
It's difficult to account for these things because the same time these new variants started exploding is the exact same time we shifted resources from random testing to mass vaccination.
Seasonal viruses often peak before the official end of winter. The key date may be the winter solstice in late December when sun exposure reaches its minimum.
