True, Wii does resemble cheap plastic in comparison, but the reason for that would be the layout of the GC controller to the Wiimote. They set out the huds in conjunction to ruffly where the buttons actually are under our thumbs/fingers. It would be a little distracting at 1st maybe for new console users (overall new console users, i.e. people not really used to ANY control scheme never mind Wii), if the 'A' button was shown on the left side of the screen in the bottom corner, while the 'B' was top right corner, and direction pad was mid right hand side of the screen.
With the huds how they are it saves beginners you having to LOOK down at their hands throught their introduction to the game, which could lead to them taking unnecassary hits, etc. Reason your brain sends the control layout map to your fingers/thumbs, and you eventually follow out desired actions quicker and easier by doing so. Same as typing without looking at the keyboard, you learn to do so quicker if you let your fingers find their way and not look at the keys when practicing. The onscreen hud, basically the control pads layout is just there for a prompt when the beginner forgets where a certain button is, or generally needs to see what item is assigned to what button.
I AM aware however you was more or less refering to the design they placed behind the buttons for the GC hud, which is true, they could of put such a pattern around the Wii hud, and maybe they will or have for the final build.