US government actions are probably also to blame for the cases in Afghanistan & Pakistan. The CIA setup a fake vaccination clinic in their effort to catch Bin Laden [1].
Another factor is just that eradicating polio with the current vaccines is hard. The oral vaccine mutates in around one in a million cases to become effectively wild polio (some years this accounts for most of the polio cases). The fully inactivated injected version is non-sterilizing: it prevents poliomyelitis but still allows for gut infection: eradicating the disease but not the virus.