Smallpox was able to be eradicated because it’s a disease that people only get once, and which does not have an animal reservoire.

The smallpox vaccine was intradermal, rather than intramuscular. Resistance to inoculation was reduced because it was just a scratch, rather than an injection.

The elimination project concentrated its efforts on outbreaks. Dr. William H. Foege developed a “ring vaccination” strategy to target vaccination around outbreaks rather than mass vaccination.

They did mass vaccination too, at least here in India. They stopped before I was born, but my elder brother got the vaccine long after we'd had the last case of Smallpox in India.