First, we want fewer nukes. Second, the U.S have allies in the region and yes, Israel is top of mind but there are others. They don't want Iran to have nukes either they just don't express it publicly. Third, we do not want to be enslaved by China and the deranged proxies they employ.
We constantly hear that Iran is the greatest funder of terrorism, this is not entirely true in my opinion. China is the greatest funder of local terrorism around the globe be it via spreading poisons that kill citizens, compromising politicians across governments (her discreetly I might add, the want all to be anti-American), encouraging open borders to destroy communities, the U.S and its allies.
Thus, it is time for Chinas global influence to wane and be challenged. Far better to do it now than when another insane regime have nukes.
Look, I hate war. I am almost as much an enigma as Trump. I operate on ptincipke, not choosing sides in totality but always reminding any side on an issue I support "look in the mirror, who must we be"?
Iran cannot have a nuke. Is it the pretext for a larger agenda? Perhaps. Thst is secondary in some respects (argued it can be primary in others). The statement still stand though. If Chinas influence and ability to drive their global authoritarianism is further undermined, that is just a huge bonus.