My own thinking on this matter includes this sort of stuff:
Seasons make a difference in a few ways. Some may be down to individual health variations during different seasons, some down to UV light destroying the virus in the environment more quickly and other environmental factors, some of them are down to differences human behaviours, eg ventilation issues and meeting indoors.
However these differences only become key difference-makers if things are more delicately balanced between conditions that can cause a wave and those where the virus struggles to get beyond a basic foothold.
Factors that go into how such balances work out include the transmissibility of the virus, human behaviours, and the population immunity picture.
Applying those things to flu, we could suggest that the flu virus doesnt spread as easily as the current covid strains, and that there are prolonged periods where it faces a far more disadvantageous population immunity picture that thwarts its ability to find enough victims to gain explosive growth. So most years (ie years where there isnt a 'brand new' new flu via a new flu pandemic) it struggles to explode until it gains the additional opportunities offered by winter, coupled with just enough ability to evade prior immunity in the population at the time. Depending on the exact state of the populations immunity and how the virus has evolved between that winter and the previous one, we either get a mild or severe epidemic wave in winter.
Clearly the balancing act is still very different with Covid at the moment. Its highly transmissible. It doesnt currently need the winter advantages in order to find enough victims. And there is no solid wall of immunity that thwarts it at this stage, for a bunch of reasons including the speed at which it is still evolving, and the failure of prior Omicron infections to provide strong immunity against future Omicron infection. And this year, unlike the previous 2 years, we didnt have a strong lockdown earlier in the year which pushed the number of infections down to a low enough level that it would take quite a while to substantially bubble up again.