The Wii runs at 480p max, so on top of the compatibility issues, the text would probably be very hard to read if XMB was somehow ported. It would either be very large (creating a “zoomed in” effect, harder to navigate lists), or small and aliased like RGUI with less contrast.
The Wii is also commonly used in 240p mode with CRTs, so XMB would really fail to perform at that resolution. It makes things simpler to use RGUI, since it works at all kinds of weird/small resolutions.
You can change the colors of RGUI, I think, so maybe you can experiment and find a palette that’s more to your liking. The Wii isn’t suited to LCD TVs, though—never was—so I’d also recommend an RPi if you want RA on a modern TV. The digital output will look better and carry less latency, and the footprint is negligible.