I really don't know enough about immunology to answer that -- but I presume most neutralizing antibodies wane over time.

If you take antiretrovirals you're very unlikely to become infected with HIV, even if you take it after initial exposure (but do your own research on that), but as soon as you stop, you lose that protection -- same with antibodies (not sure how a vaccine would help T-cell immunity if HIV infects T-cells).