I am nearly 100% confident that this decision was based primarily on the boosters effect on "public confidence in the vaccine."

That is: they don't want to push boosters now because people will use this rejection to discredit "conspiracy theorists" who say there will be endless boosters if we give in to the tyranny. And many people on the fence who don't want the vaccine if they think they'll be required to keep taking it for it to remain effective will get it when they see this firm rejection.

However, I am nearly 100% confident that once a larger percentage of the population is vaccinated, they will announce the need for boosters for everyone. And the cycle will begin anew.