How certain are we about the low mutation rate? Since many millions of people get the flu each year, there are a lot of opportunities for mutation. Also, perhaps it takes a long time for a mutation to spread widely enough for us to be aware of it. Would it already be apparent if some mutation of coronavirus existed that overcame resistance and allowed reinfection? It seems like the selection pressure in favor of such a mutation would be weak when very little of the population is immune.

SARS-COV-2 is made of a single long RNA segment.

The flu has eight segments. With that you get a substantially higher rate of recombination.

Corona viruses are slow to mutate in general, and flu is especially fast.

That's why you need a new flu vaccine every year - every year a lot of time is (was?) invested in tracking what flu mutations are appearing and trying to predict which ones will be the main ones next year. Once they've determined what they think are most likely, the vaccines are developed for those. Happily flu mutations mostly impact the protein structures sticking out of the virus wall (the HxNy is an actual description of these proteins, hemagglutinin and neuraminidase), so it's mostly a matter of retargeting an existing set of vaccines rather than creating a whole new vaccine from scratch.