Because if some day push comes to shove and the US wants to attack Iran, NATO will have to follow along because of their ties with the United States. And you cannot buy weapons from a potential enemy. Basically, for the same reason that Pakistan does not buy Indian weapons, or the United States does not buy weapons from Russia directly, etc.
That's already based on the very unrealistic assumption that Iran could offer them a deal so profitable that would tempt them to sell weapons to Iran.
Plus, Iran still has an open lawsuit against the UK for failure to deliver weapons purchased before the revolution.
Yes but UK is out of EU. EU is building their own Military not to rely on NATO. NATO is practically dead that is why AUKUS was created to confront China. EU is promoting their own Euro as the currency, will have their own military and are using their own payment system. EU would be pretty independent of the US & UK in the next few years. EU has sold arms to China in the past which is the biggest treat to the US in this century. I am very positive that they would sell arms to Iran as well. EU desperately needs Oil and Gas.