more generally, it's likely due to greater (indoor) distancing, of which school closures is a subset. the slight differences in transmission characteristics between flu and covid (also represented as differential r0) likely makes flu relatively more susceptible to reduction by distancing.

and distancing is a first-order mitigation that overwhelms the effects of inferior/ineffective mitigations like cleaning, sanitizing, mask-wearing, extra ventilation, etc. those secondary mitigations can make more of a difference when we want to pack it in, like at a movie theater, but otherwise they don't do much because those circumstances are relatively rare.

folks just don't want to accept such a simple answer because it means there's not much to hang their anxiety on, and that cognitive dissonance is unacceptable, as it always is.