In fact, every other signatory opposed the US withdrawal. However, at the time of withdrawal the US immediately imposed severe economic sanctions and threatened huge "secondary sanctions" against any country that continued trading with Iran. Willingness to enter into that sort of confrontation with the US was in short supply.

Iran rolled back all discretionary JCPOA access gradually over 18 months. Then in 2021, they passed a law that prohibited any short-notice access and most monitoring cameras.

Trump stated at the time that the agreement was "insufficient" in several areas, including no delivery systems restrictions, the existence of some "sunset" provisions, and so forth.