Why does the US say it will use its veto power in the United Nation's Security Council to block Iran from buying Sukhoi 30 fighters although it wasn't mentioned in Iran's nuclear deals.
The simplest answer is balance of power considerations. As a modern fighter, the Su-30 would dramatically increase Iranian military capabilities vis-a-vis other states in the region, especially U.S. allies like Turkey, Saudi Arabia and Israel. The U.S. has a national interest in preventing the emergence of a regional hegemon in the Middle East, or even in a single state like Iran, from being able to militarily dominate other states in the region.
While Iran is particularly susceptible to targeting by the U.S. for capability denial—because Iran has positioned itself as a geo-strategic competitor to the U.S., and the U.S. considers Iran one of the leading state sponsors of terrorism—Iran is not alone in figuring into U.S. balance of power calculations. As the U.S. is the main supplier of weapons to states other than Iran in the Middle East, the U.S.' action usually takes the form of refusing sales, but the U.S. tries to maintain a military balance in the region. The U.S. made calculations about Saudi Arabia's power vis-a-vis Israel before selling Saudi Arabia a weapons package. The U.S. has held up a Qatari weapons deal, in part out of concern that it will upset the balance of power. Even Israel did not receive a delivery of Bunker busters until after the Iranian deal was signed.
The signing of the nuclear deal is immaterial to the sale of advanced fighters. The nuclear deal is an attempt to stabilize the regional nuclear balance of power, but it says nothing about the regional conventional balance of power. While conventional war is—hopefully—less likely because of the nuclear deal, it remains a potential outcome, and U.S. leaders continue to plan for that contingency. If there is a diplomatic tool which will make a conventional war easier to win, the U.S.—and probably any government—is likely to exercise that tool, even if they are hope that a conventional war does not occur.