Using ASCII in URLs is simple and is less error prone than “supporting” unicode via percent encoding. It is also just a convention to use ASCII for usernames in many platforms. ASCII is also supported out of the box in major OSes while some unicode characters might not. What about impersonation? And what about people trying to type in the username of someone that uses unicode? It is not logical to use unicode in this case.
99.9% / 0% / 0.1%