I imagine the real reason why a Pope from the U.S. would not be a popular move is that it’s not good to have such a close alignment between the world’s only superpower and the head of the Catholic church. The rest of the world, particularly the poorer countries, would wonder whether their needs were truly being met or understood. Often, American Bishops aren’t as fluent in other languages, and so their understanding of other cultures and the truly global nature of the world can also be limited by that.
On the other hand, there had also been long periods where the Pope was often Italian. Makes some sort of sense since he is the Bishop of Rome. But from the world’s perspective, can an Italian truly understand other aspects of the rest of the world? So obviously there are cases to be made for why a Pope from the third world, or not from Italy, or not from U.S. would be good for the global church as a whole.