note that it’s not that the flu virus mutates so much more than other viruses, as it’s using the same (imperfect, mutation-inducing) machinery to replicate itself as other viruses, it’s that its genome is sufficiently resilient to mutations that more variations can survive to infect others. perhaps there are many more, or just more flexible, non-critical regions that can be swapped out willy-nilly without damaging the core replication function. or the replication-coding regions of the genome can be swapped around more easily without ill effect. or there’s more error-correcting. or a number of other plausible mechanisms than being better at mutating, because that’s unlikely.