Chris Preble, vice president for defense and foreign-policy studies at the Cato Institute and a friend of mine, has written in The National Interest that there is reason to believe Iran had ceased its research on nuclear weapons years before the JCPOA because it was simply too much trouble. He writes (abridged):
It was never obvious that Donald Trump knew much about the Joint Comprehensive Plan of Action (JCPOA), the nuclear deal with Iran, but he knew the important parts: It was Obama’s deal. It was a deal with Iran. Those two crucial facts likely doomed it from the start. Trump called it the “ worst deal ever” negotiated.
Given his disdain for the JCPOA, it is somewhat surprising that he stayed in it as long as he did. He was likely swayed, temporarily at least, by Secretary of Defense James Mattis, and then-Secretary of State Rex Tillerson. Both men urged him not to withdraw. The State Department had affirmed, on numerous occasions, that Iran was in full compliance with the terms of the deal, as did the International Atomic Energy Agency, the entity entrusted with monitoring Iran’s nuclear activities.
There is reason to believe that Iran agreed to limits on its peaceful nuclear program and ceased research on nuclear weapons years before the JCPOA ever went into effect because Iran’s leaders concluded, as the vast majority of countries have, that nuclear weapons simply aren’t worth all the trouble. As noted, under the JCPOA, Iran “reaffirm[ed] that under no circumstances will Iran ever seek, develop or acquire any nuclear weapons.”
Read more here.