Unfortunately, the U.S. only restriction is due to the varying laws covering giveaways around the world. Some countries ban them entirely, some require an element of skill, some require selection by chance, some have language requirements (looking at you, Quebec), etc.
In short, it's a legal nightmare to even attempt to comply with laws in all of these countries, and we simply don't have the resources to manage it. We really wish we could, because we know we have many loyal readers around the world, but it's just not feasible. You'll see similar policies on many other major media sites that do giveaways.