Also, smallpox doesn’t jump species, it’s only been found in humans. Plus, It’s not extremely contagious like measles or even covid19, although someone who sneezes or coughs on you can transmit it, it doesn’t easily float through the air for long periods or distances.
It’s hard to eradicate a disease when it is harbored in animals. If we keep humans away from the animals that harbor the disease we can reduce the risk of another outbreak if we actually get rid of the virus at least temporarily among humans, and if we have a vaccine that increases immunity that can stop the virus from traveling across the human population when there is contact.
If someone released smallpox today, more than 50% of the population has zero immunity. The last people to get vaccinated are in their early 50’s now. If you’re under 50 you have no immunity, unless you’re lucky enough to have natural immunity (rare) or you are part of the few thousand in the world who are vaccinated for special circumstances.
Measles and polio don’t cross species either. That’s why the Gates foundation is making an effort to rid the planet of both.