The United States throughout the years and decades has imposed sanctions on the country of Iran and we have seen some results of this. Now Iran is finally sitting at the table with the international community and is willing to talk. Further sanctions against the nation will more than likely stop talks.
When it comes to Iran, I believe people think they're close to nuking everyone including Israel, which isn't true at all. Iran hasn't done anything for them to have sanctions, people can even visit Iran with no issues by the Iranian government. So if they needed greater sanctions then they should also have a no fly zone for that area.
I believe this issue comes from the former Iranian President making the statement about Israel with wiping them off the map, yet people have forgotten that he only said that statement on if Israel were to strike them first. It was never said that he would just strike Israel.