It's probably because they are on the clock and the vaccine isn't, at least from what I've heard from other places, being administered yet at places of business so they'd have to leave work (or not be available if doing remote teaching for those with school back in session or will be in session when teachers are up for vaccination).
Plus with the side effects they've, from the beginning, advised 1 to 2 days off work afterwards, it's one of the main reasons they have mostly chosen to stagger the health care workers (which of course slows down an entire place having the employees they choose to get vaccinated).