So I guess you need to explain exactly why Russia, China and, well - there isn't really anybody else with the delivery systems to strike the US - would be willing to risk MAD over Iranian nuclear facilities being taken out by nuclear bunker busters.
The mad thing was just in response to your assertation that the US is inviolate when it comes to nuclear attack, which is bullshit.
As for why, they won't, they don't have to. Nukes are taboo. Ever since WW2 ended, there's been sort of an unspoken agreement that we don't use nukes. Yeah, they exist, and nations in conflict may have them on both sides, but when that happens it's all diplomacy and brush fire wars in out of the way places so we can pretend that we aren't really at war. It's not that different from how gas just wasn't used as a weapon in Europe in WW2. Everyone was afraid of it, gas masks were handed out like candy, but everyone had seen it, seen the results and knew just how fucking awful it was and wasn't willing to go there. We have seen nukes, we get radiation, and everyone does their best to make sure it just doesn't happen.
If the US nukes somewhere, anywhere, without a direct nuclear attack on US soil first, they have broken that unwritten rule, and every other country on the planet realises that they can't predict or anticipate how the US might act or react because they are using a completely different playbook to everyone else.
Countries like Canada and Australia, strong allies to the US, resource rich, little military capacity and relatively isolated from meaningful assistance from anyone have to consider that the US may now stop 'asking a friend' and start 'telling a servant' or may just roll in and take what they need if they feel they have to, small island nations or middle eastern countries selling oil to the wrong people have to consider a marine division might just rock up and take over one day because they have the rights to some pissant little oilfield that no one paid much attention to before.
Europe will be looked to to try and check this crap, and realise they have no real capacity to do so, and as a result, everyone will start gravitating to the one nation that can get withing throwing distance of the capacity to do so sometime in the foreseeable future (excluding maybe India.) Sure, as it stands now none of this is at all likely to happen (except the invading little countries for oil bit) but once one big rule is broken you have to assume that the whole damn rulebook has been thrown out and react accordingly. Opening this particular Pandora's Box makes you the bad guy.