Poliovirus and smallpox infect only humans and have no intermediate hosts, as is not the case with the flu that cross-infects birds, horses, pigs and humans as of today and then who knows. If there are no new victims to infect, viruses like polio and smallpox cannot longer survive in the environment because they are not capable of adapting to a different host. When coupled with their almost non-existing mutation potential, mass vaccination and containment were the major driving force in providing us with the life long immunity and complete eradication of the smallpox (nearly there for the polio).
That is not going to happen with the Influenzavirus. Being an RNA virus, it has high mutation rate and can infect the same host over and over again because the specific antibodies produced following flu infection or vaccination ONLY prevent subsequent infection of the original non-mutated virus - one reason why flu vaccines sometimes fail. Second, RNA genome of the flu virus often recombinates with other flue types or even those from other species resulting in hybrid viruses. Again, the new flu strains appear before the adaptive immune response is formed and/or vaccines are made.
FYI: The RNA retrovirus HIV (causes AIDS) has the highest mutation rates ever AND goes into the latent phase by hiding out in the resting T lymphocytes thereby completely failing an HIV vaccine as yet.
Which also explains why mutts make the best dogs.