The simple explanation is as most people respected the stay at home ordered, specially around Christmas, and alot working from home, and no gathering in bars and restaurant (no Chrismas' office parties). And most people wearing masks, no crowdy shopping's mall. It just prevented the flue virus to circulate and infect us all.This year there was no flu season.
I don't know why, unless those (like Sol) who think the coronavirus is just the flu, by then we have a really long flu period for 2020-2021