The thing about public health is that it's a game of averages. Smallpox vaccination, because of the specific details of the vaccine itself, has significant potential side effects well beyond any other modern commonly-administered vaccine. These side effects can be permanently disabling, or in some cases even cause smallpox-like illness itself. When smallpox was widespread, the downside of vaccination (side effect risks) was much smaller than the downside of not vaccinating (dying of smallpox). Now that smallpox has been eradicated, the calculation is changed - on average the damage that will be done by side-effects in a wide-spread vaccination campaign are outweighed by the extremely small risk caused by not vaccinating.

That leaves the question of whether a better (safer) smallpox vaccine could be developed - I don't know enough about medical science to know if that's feasible, but even if it is it's likely not worth the cost and effort to develop a new vaccine for a disease that is no longer really a threat.