I was wondering why other viruses like Chicken Pox or Measles haven't mutated into terribly contagious or dangerous strains? Why haven't there been variants of these viruses that have required new vaccines?
Becuase the flu is unique in this regard, it divides and recombines as a matter of course, that's why they number the combinations, like h1n1, h1n2, etc. Most other viruses, including coronaviruses, don't do that like the influenza does.