because we chose to show mercy.
Because we didn’t want Iran partitioned between US/USSR/Iraq and possibly WWIII starting there.
RR may have joked about WWIII but, he was carefull not to let things get out of hand.
The unofficial agreement with the USSR from 1979 on was that if we went to war with Iran they would re-occupy the north while we took the south. We did not want the USSR to have a border with their treaty ally Iraq - that would have put the USSR in position to take the entire GCC.
Mercy had nothing to do with it...