It [the flu] is not absent; it is reduced. I pointed to this in this post at the bottom where the respiratory deaths are listed for 2019 and 2020. The flu season varies from year to year anyway. Coronaviruses, all of them affecting humans including Sars-Cov-2, and influenza viruses are spread via the same mechanism, airborne.
Is it really that surprising that measures to reduce airborne transmission or one virus will reduce another?
Social distancing, hand washing and gel use, masks, closures of indoor gatherings and all the rest will reduce transmission of BOTH influenza virus and coronavirus and any other airborne virus too.
There are other factors. The flu vaccine uptake has been higher this year than any other. There is more informaton than you could need in the above link.