I doubt that it was the only sample in the world to escape eradication.
https://www.wsj.com/articles/SB1039129770495563833
https://www.nytimes.com/1999/06/13/world/government-report-s...
I think it's just that nobody has both the ability and desire to destroy the world at the same time. Do we even know if smallpox would kill a modern human?
The last person to die of smallpox died in my lifetime. I hope I'm not so old that I'm no longer considered a "modern human".
Plus, effective heard immunity requires something north of 85% of the population. Smallpox vaccination rates are not even 50%.
Smallpox on the other hand is much more likely to just get your home country nuked into oblivion if they find who deployed it. It's effectiveness as a weapon is going to be limited in first world countries with ring vaccination programs and epidemiologists. On the other hand it would be devastating to the countries that would have a ideological reason to deploy them.
Modern terrorism is a game of chicken. You want to be successful enough to degrade the enemy without triggering a backlash. Smallpox and nuclear weapons are great deterrents, however deploying them would mean the end of the countries and organizations using them.
Dr. Strangelove: "Of course, the whole point of a Doomsday Machine is lost, if you keep it a secret! Why didn't you tell the world, EH?"
Others in comments also made a good point...what if the virus survives decades in cemeteries and is released when they dig or move the cemetery. Within a week hours it would probably be spread all over the world, given globalization.
After all, the goal is usually to get the enemy to surrender / capture available resources / not turn the rest of the world against you. Millions can certainly die, but it could be very possible say for a World War III between US and China where no one winds up using nuclear weapons.