Juan Cole explains it all for you:
Iran has 150 billion barrels in petroleum reserves, among the largest reserves in the world, but they cannot be exploited by US corporations because of Israel lobby-inspired US congressional sanctions on Iran. US elites, especially Big Oil, dream of doing regime change in Iran so as to get access to those vast reserves. Likely the most important US objection to the Iranian civilian nuclear enrichment program is that it could give Iran “nuclear latency,” the ability to construct a bomb quickly if it seemed to Tehran that the US planned to attack. That is, the real objection in Washington to Iranian nuclear know-how is that it makes Iraq-style regime change impossible and so puts Iranian petroleum out of reach of Houston for the foreseeable future. This consideration is likely the real reason that Washington does not, so to speak, go ballistic about North Korea and Pakistan having actual nuclear warheads, but like to has a fainting spell at the very idea of Iran enriching uranium to 3.5 percent (a bomb takes 95%). North Korea and Pakistan don’t have oil.