The best explanation for the eradication of small pox is that the disease was eradicated due to a massive global immunization campaign led by the World Health Organization.
The last known case was in Somalia in 1977. Since then, the only known cases were caused by a laboratory accident in 1978 in Birmingham, England which unfortunately killed one person. It caused a limited outbreak.
After the global immunization program by the World Health Organization, small pox was declared eradicated in 1980.
