Reference

The UN Security Council rejected the US demand regarding Iran sanctions due to several key actions and circumstances:

1. US Withdrawal from the JCPOA: The US, under the Trump administration, withdrew from the Joint Comprehensive Plan of Action (JCPOA) in 2018, which was a multilateral agreement that provided Iran sanctions relief in exchange for limiting its nuclear activities. This withdrawal was a significant factor leading to the US's subsequent isolation on the issue within the Security Council. Source

2. Attempt to Extend Arms Embargo: On August 14, 2020, the United States proposed a resolution to extend the global arms embargo on Iran, which was set to expire under the terms of the JCPOA. The Security Council rejected this resolution, with only the US and the Dominican Republic voting in favor, while Russia and China opposed it, and the remaining 11 members, including US allies, abstained. Source

3. "Snapback" Mechanism Invocation: The US attempted to use a "snapback" mechanism to reimpose all previous UN sanctions on Iran, arguing that as a participant in the original JCPOA agreement, it could trigger this provision. However, the majority of the Security Council, including US allies, did not support this move, viewing the US withdrawal from the JCPOA as disqualifying it from invoking the mechanism. Source

4. Diplomatic Isolation: The US found itself diplomatically isolated on this issue, with even traditional allies opposing its stance. This isolation was a result of the US's unilateral actions and the perception that it was undermining a multilateral agreement that was seen as crucial for regional stability. Source

These actions and the international response to them led to the UN Security Council's rejection of the US demand to reimpose sanctions on Iran.