Errr - because they will be a sink of infection that will remain a threat to (1) the vaccinated where the vaccine is not completely effective, and (2) those who cannot be vaccinated for medical reasons. Oh - and the longer the vaccine remains in humans the more chance of mutations that may make the vaccine less effective.
Thus, the unvaccinated remain a serious threat to humanity (only a slight exaggeration for effect).
OK if they won't take the vaccine after watching everyone else take it then we could just shoot them and radically improve the gene pool.