Xbox is only selling in the USA, except the sales it has in india, and the UK and many other countries all over the world. Maybe you meant to say selling best in the U.S. or the U.S. is its strongest market? There's no way there are no Xbox Series X|S consoles being anywhere else in the world.
I know I'm taking you literally, but there's not much wiggle room with what you said. Microsoft has to satisfy demand all over the world as well, same as Sony. There have even been Xbox Series consoles sold in Japan.
Funny these kinds of excuses are never made when Playstation outsells Xbox. To my knowledge, the Series X launch countries were many more than the Playstation 5s, were they not? If I'm mistaken on that, feel free to correct me.