There are two reasons. The first is that the US, and indeed the world, believs that these countries possess a greater desire to use these weapons than do others. If they got a hold of one, they would only use it for their own political or military ambitions and would not hesitate to use them against civilian targets.
The other reason is that many of the countries that have these weapons, we would not allow them to had they not already gotten them. I can guarantee you that if the US had a choice, then we would choose to keep the weapon for ourselves. However, when countries like Russia and China got them, there was little we could really do to stop them. It would be crazy of us to invade an allied nation like Israel or Great Britain in order to destroy their nuclear stockpiles.
