The first reason is that Iran, despite media reports, is not about to acquire a nuclear weapon. As the IAEA report indicated, there has been no diversion of fissile material from known facilities inside Iran, and without fissile material that the Iranians could enrich to the levels required for a nuclear weapon, they cannot go nuclear. Iran would need either to throw out the inspectors and divert their existing stocks or else develop very large enrichment facilities without their being detected by the outside world. Throwing out the inspectors would be easily observable, and it is unlikely that Iran has covert facilities sufficient to make it a nuclear weapons state any time soon.