It seems counterintuitive at first, but the logic is sound. More infections = more mutations. More time = more mutations. By having a significant proportion of unvaccinated people keeping the pandemic going, juxtaposed with a large, maybe even majority of people vaccinated, it confers an evolutionary advantage to any mutation that crosses from unvaccinated to vaccinated. A rapid increase in vaccination is needed to stamp out the infection before it can mutate. A slow burn is a worst case scenario, as it just keeps the door open for the shit to keep escaping.