To understand why the Nuclear deal was cancelled, you would need understand the political school of thought which considers any concessions to the enemy short of unilateral surrender from the other party to be a loss and showing a sign of weakness. These are generally Nationalist hardliners and exist in every country. Examples would be South Koreans who consider any negotiations and granting concessions to North Korea to be an abomination, or Indians staunchly opposed to giving any concessions to Pakistan believing inherently their enemy to be untrustworthy and evil. The same is true with US hardliners and it’s stance towards the “Axis of evil”.

The long and winding road with Iran

Among this Axis, Iran holds a special spot of distaste amongst the hardliners. Not only does Iran invoke imagery of hostage crisis but it also went on to turnaround war against Iraq despite significant US support to Iraq and become a regional superpower clashing with US allies Saudi Arabia and Israel and threatening US energy interests in the region. Successive Presidents ever since FDR have resolved to protect “vital interests” in the Persian gulf region and President Carter laid it out on his doctrine:

Let our position be absolutely clear: An attempt by any outside force to gain control of the Persian Gulf region will be regarded as an assault on the vital interests of the United States of America, and such an assault will be repelled by any means necessary, including military force.

Even though intended for Soviets messing around with Afghanistan, the “control of the Persian Gulf” was indeed taken by an “outside force” shortly after- Khomeini’s crew in Iranian Revolution. This instantly created 1979 energy crisis even triggering minor recessions in US exposing strong dependence on Middle-Eastern oil reserves. Thus US strengthened it’s alliance with Saudi Royal family. But not only did Khomeini end there, he promised to overthrow capitalism, American influence and monarchies alarming it’s smaller Sunni neighbors including Saudi Arabia.