The Ayatollah issued a fatwa, decades ago, prohibiting the development of nukes.

Not good enough says Trump

Iran is a signatory to the non-proliferation treaty (iircIsrael is not)

Not good enough says Trump.

Iran signed the JCPOA

Not good enough says Trump.

You're making the Iranians out to be saints. They're not.

Trump ripped up the JCPOA. Incidentally, why did he do it? Main reason, it was Obama's agreement. Second reason, Beeb talked him into it because Beeb fought it all along on the premise that it didn't go far enough. The only agreement good enough for Beeb is one where the Iranian regime commits suicide. Anyhow, Iran waited more than a year after Trump ripped up the agreement, and then they tore up their own copy and proceeded with a massive enrichment program to 60%, which has only one conceivable purpose - to build nuclear bombs. HEU at 60% can be further enriched to 90% within a matter of weeks. At that point no one in their right mind could trust Iran that they're NOT building a bomb. Trump of course doesn't have the skill or the patience or the credibility to embark on a new negotiation with Iran that could take years, with zero chance of a more favorable result than the JCPOA, so here we are.

As a corollary, I actually don't think Iran having a nuclear bomb is the end of the world. This nuclear arms race started with Dubya declaring the Axis of Evil made up of Iraq, Iran, and North Korea, and then proceeding to invade Iraq. The N. Koreans and Iranians concluded that they need a nuclear deterrent to survive. N. Korea did it, and it paid off because they became untouchable. Iran wavered but certainly decided to become a threshold-nuclear power to gain leverage. Similarly, India and Pakistan both got nukes as a mutual deterrent, and it works between them. If Iran builds a bomb it will give them the same kind of deterrence, but I don't think they'll ever use it. Its main effect will be to limit Israel's and America's options in the region, and they're both willing to go to war before that happens, proving to Iran that it really needs those nukes.