Not vaccines.
They were given in early 2021, a year before the tide rolled out, and even their backers concede they don’t work well against Omicron.
No, what happened was Omicron itself. It swept through the United States in the winter of 2022, the vaccinated and unvaccinated alike. Its mortality rates were overall much lower than the earlier variants - probably in the range of 0.05-0.1% - though its transmissibility meant that the winter of 2022 still saw a lot of deaths (particularly in older people, no matter that they were jabbed).
Ever since, various not-very-lethal Omicron strains have bounced around a population that has real (aka natural) immunity"
Edit:
The numbers don't lie. Take a look at them for yourself. The chart on page 23 is what Insurance companies will base their policy rates on in the future. https://www.soa.org/49b504/globalassets/assets/files/resourc...