The flu and other seasonal viruses are what scientists call endemic diseases, which means they’re constantly around and making some people sick. It also means a sizable segment of people will have some amount of immunity to them that might prevent infection or make reinfection milder. For some diseases, it’s thought their timing is influenced not just by weather conditions but by the level of immunity in a population (this may explain why some viruses tend to surge every two years).
Covid-19, unlike these other diseases, is brand new, so the baseline level of immunity is much lower than other seasonal respiratory viruses. There is evidence emerging that some people may have a pre-existing T cell immune response to the virus, borrowed from dealing with other common cold coronaviruses. But it’s not yet clear if and how having this response affects a person’s interaction with the virus, including their risk of catching and spreading it further. In any case, it’s still true that most of us haven’t encountered this specific coronavirus, and that likely greatly dampens any potential effects of seasonality.