Hospital Capacity.
I live in North Texas, our Trauma Service region has 4m people in it, at one point during the Delta wave we had less than 70 free ICU beds - and we have more hospital capacity than much (if not all) of the state per capita (and indeed, much of the country as well). During the Delta wave, the hospitals here were full of unvaccinated 20 and 30 somethings without complicating health issues, with serious COVID, most of which would have been prevented had they been vaccinated.
Unless the social contract is changed so those who are unvaccinated can be denied treatment (meaning limited to palliative care) during periods of capacity required rationing for serious (read, requires an ICU bed) cases of COVID, mandatory vaccinations are effectively a requirement.
Is it fair to the person who was vaccinated, then gets in a car accident (or has a stroke, or some other issue) and dies waiting for an ICU bed, because all the beds are full of folks who could have been vaccinated (and probably wouldn't have needed an ICU bed if they had) but decided not to be?
I'm pretty libertarian, and I loathe making these choices - but as long as hospital capacity is a shared resource, your choice as an individual effects others, and in times of crisis society sometimes has a right to decide some things for you, whether you like it or not, remove that linkage, and then folks should be able to do as they please.