Nuclear fallouts, enviroimental problems the nuclear cloud can fly trough atmosphere, causing cancer and other nuclear related diseases , the soil gets unfertile,water gets poisioned and you have to keep hide in case of rain also the defense chance is lower in a nuclear bombardment.
also, nobody would like to be hit by one....i wouldn't like to have São Paulo Nuked, Nor you would like to have NYC nuked.
Back in the day people had no idea of what nuclear weapons were capable of in a city, but now we have some notion and i think that's an international concern that Nuke somebody is not a good thing to do, at least it's not 'human' , but hell, everybody uses chemical weapons also.
Chemical warfare got the same repulse after WWI that nuclear weapons got after WWII.
it's an evil weapon by definition, dresden/tokyo incenriary bombardments weren' t worse because of the side effects caused by ratiation.