I think part of the reason it seemed unusually stable was that it was operating in basically a vacuum for immune evasion pressure. Every host was a naiive one without prior covid exposure. There was therefore not as strong of a selection pressure as the other coronaviruses were under to evolve new variants that could evade prior immunity. Once you get that selection pressure, the number of apparent new variants ratchets up quickly, because anything that isn't new can't spread effectively in our high-immunity environment.