Earlier quoted context omitted.
The likely explanation is that, because the flu is significantly less transmissible than COVID, even states with "no restrictions" were able to knock it out because enough people took enough measures to protect themselves from airborne/surface communicable disease in general. In other words, you can probably beat back the flu significantly by having enough of the population mask up, wash/sanitize their hands, etc.
But the flu has always been here, even when our country was significantly less populated, when we were significantly more rural, and when people were significantly less connected in many ways. Like, how did we get flu pandemics in the 1700s? Weren't we less connected back then, even after you account for all the social distancing that was done this year?
Germ theory of disease didn't exist back then.