Because most of the effect of vaccines is their effect en masse. It's a solution lying almost entirely within the realm of collective action, so it's a test of whether we're collectively smart enough to implement a rational collective solution, which includes using some degree of coercive measures to achieve it.

Basically, if we let a pandemic kill millions because a fetish for personal choice prevents us from doing what's necessary to prevent it, we're too fucking stupid to survive as a species. That doesn't have to mean going to door-to-door with guns and needles; getting to the critical mass we need just means refusing to let the stupidly unvaccinated into a variety of public venues like airplanes and universities long enough to make them get the vaccine.