Probably had to do with casting Americans in rebel roles and Brits in the bad guy roles. If the casting had been reversed, the intent would have been more obvious, and the US box office a bust.
Wedge was Scottish, at least in Return of the Jedi, and I think Red Leader was Scottish as well. Vader was also voiced by an American.
So far as your theory, actually, I think even that wouldn’t have made the intent more obvious. What WOULD have made the intent more obvious is if they cast the Rebels as Vietnamese people, though your point about Americans in the bad guy roles definitely works for the intent.
Anyways, it’s largely because of how the Rebel Alliance was inspired by the Vietcong as well as George Lucas’s bombshell about how Star Wars was not only anti-Vietnam War, but also pro-Vietcong propaganda that I’m an Empire supporter now (and I’m not even particularly happy that I have to support the Empire as of now, but thanks to that, my love the Rebel Alliance has been destroyed, possibly permanently, wrecking a large part of my childhood as a result.). I simply cannot in good conscience support something that was explicitly based on a Vietnamese communist rebel group backed by the hated USSR that, alongside other communist groups and even other atheists tried to wipe us Christians out (in case you’re wondering about that other atheists bit, the French Revolution did similar stuff and that predated Karl Marx’s birth), and that combined with the fact that George Lucas made the country that, if anything, tried to SAVE Vietnam from becoming a anarchistic/communist totalitarian nightmare, into a villain left me with a lot of disgust for the Rebel Alliance I had formerly looked up to as a kid, a naïve kid who believed they were the American Minutemen. Essentially, I started rooting for the Empire because right now that’s the only faction I CAN support. I can’t even allow myself to continue to believe that the Rebel Alliance was the Minutemen after Lucas explicitly trashed that, as I’ve pretty much been taught that, no matter what my personal beliefs of what a work is supposed to represent, they are INFERIOR and MEANINGLESS before the actual creator of said work. It’s ingrained into me so much that the only way my views would ever, EVER mean anything regarding that work is if I shove said creator off a building and take control of it myself, THAT’S how deep my feelings of inferiority before a person who creates something lie. And since I’m forced by the Ten Commandments to not kill, I can’t ever let myself be that.