Why several countries are allowed to have nuclear weapons?

Why several countries are allowed to have nuclear weapon(s) and other nations are not?

Does the U.S. not care about the safety and security of other West Asia (Middle Eastern) nations? We know that Iraq’s security was not of any interest to the U.S., or it wouldn’t have destabilized Iraq by invading it in 2003. And with assassination of Iran’s nuclear scientists, there can’t be too much concern about that Iran's internal security.