I think its mostly business tinged with racism. For a multi-million dollar film like this, they have to sell it by making some assurances to the backers that they'll make a tidy profit. Using big name stars is one of the ways they do that. It probably helps pull in some customers as well. Play it safe with a well known white actor or take a chance and use a lesser known Persian/Iranian actor.
Frankly I'd rather they'd worry about the quality than image, but the world doesn't work the way I think it should