Because S-400 is a defensive asset used to defend ones territory. You cant use an S-400 to bomb Saudi Arabia or fire upon US ships in the PG.How do you know? Russian officials have said that they are ready to sell the S-400 air defence system. so I don't see why fighter aircraft would be a problem. So far it seems that the lack of action is because Iran is not ordering.
Can’t believe you are comparing the two. And also Russia only offered the S-400 knowing Iran didn’t need it anymore with its Bavar system. So it offered something it knew iran would reject.
Russia will offer Mig-31 once Iran can build something in the same ballpark. That is how Russia operates.