Compared with unvaccinated individuals, vaccinated individuals more often had homemaker service, were more often born in Sweden, had more comorbidities, and had a higher level of education at baseline (p<0·001 for all; table 1). Similar differences were evident between vaccinated and unvaccinated individuals in the different vaccine subcohorts.

Translation: people that got boosted were, on average, older and had poorer health to begin with. That's why they got boosted.