Because Iran is a member of BRICS and BRICS establishes its own payment system as an alternative to SWIFT: https://observerdiplomat.com/brics-officially-announces-fina... on top of which they have digital payment systems with globally the most users - all of that bypassing US banks, so that the US no longer has a means to control/sanction trade and payments.

TL;DR the world is leaving the premises where the US unilaterally decides which population should starve.