Because it is contagious and spreads rapidly. It is the gift that keeps on giving. Anthrax is not contagious and nukes, except for fallout, have a finite range of annihilation.
The Russians weaponized a variola that appeared in India in the late 1960s, a virulent smallpox strain with a kill rate of 50% rather than the 30% that existed elsewhere at the time. It was powdered, placed in the nosecones of ICBMs with the intention of bombarding a virgin population, namely the U.S.
Not only have we quit vaccinating during the past 30 years but those who have been vaccinated (except for military personnel) were never vaccinated for the deadly India I variola.
Letting loose smallpox in our nation would be devastating. And if you think the CDC can deal with it, look how poorly they're dealing with the West Nile Virus situation. It's pathetic.