Evolution is accelerated by divides and micro-populations, not decelerated by it.
Its why like every river system in the Appalachians has its own minnow species, and the Galapagos Islands have so many different finches. Certain traits tend to dominate smaller, isolated populations faster than they do large, fluid ones.
But viri don't need a partner to mate with, this means that in isolated communities all it takes is a single advantageous mutation to arise and then said substrain/variant could quickly dominate the viral population in that locale.
I've read conflicting or different things on it. Some say the vaccine reduces serious outcomes like hospitalization and death and also that it will reduce transmission but it doesn't seem as effective in reducing the transmission part which most vaccines are. I thought more immunity would mean less transmission but I guess you can still be a vector and spread it while asymptomatic or vaccinated? It still protects against transmission somewhat though so if everyone was vaccinated there would be immunity on the population level and less chance for it to successfully spread.
Most sources admit the vaccine is non-sterilizing. Now of course its not a binary, so obviously an asymptomatic carrier isn't going to be spreading it as much as a symptomatic carrier; but its still not enough to outright put an end to viral transmission.
This is to say nothing of the fact that COVID has animal reservoirs.
This basically means that aggressively trying to vaccinate the population with a non-sterilizing vaccine is simply selecting for viral traits that allow it to spread in spite of vaccination/host immunity.
Which, given the claims that people who've had COVID can get it again and that despite being vaccinated you can still spread the virus, implies that such traits are likely already widespread in the population and this vaccination campaign is simply selecting for greater expression of such traits.