There are a bunch of contributing factors (most schools, indoor public places, and workplaces encouraging or requiring masks, and many workplaces remote), but one thing that I'd expect to help with flu much more than it does with COVID-19 is sick people staying home.

Asymptomatic spread is a much larger risk with COVID-19 compared to the flu. A simple behavior change of "If I feel sick, I don't go to work/school/grocery shopping" isn't enough to stop COVID, but could cut flu spread a ton by itself.