Basically because the mechanisms this generation of vaccine consist of are much less complex than any before. Historically, vaccines were huge biological black

Basically because the mechanisms this generation of vaccine consist of are much less complex than any before. Historically, vaccines were huge biological black boxes with all kinds of biomachinery inside that weren't understood at all, that weren't even known. Vaccines were routinely developed, purely by clever trial and error search strategies long before any of their actual mechanisms were understood. In particular this meant that most of the molecular machinery included weren't even needed for the effect. You want a lipstick so you steal cars until you find one that contains a lipstick. Who knows what else you acquired. This generation of vaccines is the opposite, they know exactly what molecules they want, where, for what reason. And it's basically one molecule. One molecule you'd be exposed to, in much larger dosage, anyways one day if the vaccines are unsuccessful (the virus is well past flash-in-the-pan now, it will become endemic unless perhaps vaccination also prevents spread and not only illness, which isn't clear at all). The only uncertainty was the question of whether the immune system will react to the sound protein if it comes without a virus attached and wether an immune system that was already acquainted with that molecule will actually work better against the actual virus, not worse (some viruses are even worse if the immune system thinks it already has a defense and focuses on that but the defense is only partial). Both are easily checked in trials if you don't skip them (we didn't). The black box full of surprises of historic vaccines, the trunk of the car that you stole to find lipstick, it's just not there.