People usually get it at specific intervals as children. It's easy to plan ahead when you have everyone scheduled weeks in advance on a routine schedule. Not so much when you have everyone descending on clinics at the last minute.

The problem I think you're forgetting is that vaccines have a very limited shelf life. They can't just keep tons in stock. In a sense they're made to order, so everybody trying to get one all of a sudden is causing strain on a system that was never meant to deal with on-demand vaccination.

There were years that the vaccine was less effective or that they gave one instead of two. Now we're learning that we may need a booster.

Apparently the original vax used between 1957 and 1967 wasn't strong enough to produce permanent immunity, so people vaxxed in those years are being told to get boosters. And between 1967 and 1978, everyone got a single shot for their vaccination, but they later decided it should have been a two-shot series, so one-shot vaxxed from those years are supposed to get boosters.

There's a blood test that can tell if your immunity has waned, or you can just ask your doctor for a booster shot.