The smallpox vaccine is a sterilizing vaccine, meaning you cannot be infected and cannot transmit it once you are vaccinated.

The MRNAs are non-sterilizing, as are many vaccines. They may reduce symptoms, reduce infections, and reduce transmission, but they are not considered a sterilizing vaccine.

Secondly, coronaviruses mutate much faster than smallpox, and have other hard to deal with stuff like asymptomatic transmission.

The only diseases we ever eradicated were polio and smallpox. I think we got rid of the guinea worm too. Out of all the many many diseases we have fought over the years.