They picked a protein that was rapidly mutating. The vaccines were ineffective after about 3 months from release date. After that time the protein in the emerging COVID strains was completely unrecognizable to the antibodies that came from vaccine exposure.
Also picked a protein that was biologically active on its own. It also was a persistent little bastard that was produced for months not days or hours like they thought. No one bothered to measure what antibodies were elevated during clinical trials but just looked for an (any) antibody response. Also didn’t look for how long and the transition from igg1/3 to 4.