Because lots of people continue to grow and shed vaccine strain polioviruses in stool for months to years afterwards, and some strains eventually mutate to become dangerous again.
A child of the peace and antiWar movements, a Truther with self-diagnosed Opposition Defiance Disorder, formerly politically liberal tho now politically marooned, and Post-Doomer, on any issue, I trend to the conspiracy side, sort through the absurd, fantastical and insane, until I find firm ground usually located just the other side of the censorship firewall of propaganda and orthodoxy, dogma, and other either / or thinking.