What makes you think that all viruses and bacteria mutate at the same rate? This one just happens to do it quickly, which means in retrospect probably targeting the spike protein wasn't a good way to go for the vaccine had we had the luxury of more time.What makes you think the other viral and bacterial diseases don’t mutate and evolve? it’s just slower when you have fewer people harboring the disease.
Worldwide measles cases are counted in the tens of thousands. If we had had 2 billion measles cases last year, I’m sure you’d see some variants.