Because we don't have banking relations with the Iranian Government.
This issue came up again when some of the funds to be returned in an Omani Bank were to be repatriated back to Iran. The plan called for a US institution to convert the Rials into Dollars which could then be converted to Euros.
It was a big controversy as *GASP* Obama lied and actually did try to give the Iranians access to the US financial system. He did this by asking two different banks to process the conversion but both banks turned him down.
https://www.cnbc.com/2018/06/06/the-...al-system.html
