Why has Nintendo not released retail versions of all their former controllers for use on the Wiimote? Why has SEGA and SNK not put out their similar controllers and why is there no TG16 controller that can work on the Wii? Why am I destined to use adapters to play my VC games?
Well, I believe the NES and SNES controllers have now been released in some form or fashion, and the N64 controller is not very popular among most people (and there are very few N64 games on the service to begin with).
As for the TurboGraffX and Genesis/Megadrive controllers, those are probably largely up to their respective companies.
Honestly, though, as cool as the novelty is of having an SNES controller accessory, I can't see myself using anything other than the Classic Controller Pro. The Pro is just a perfect controller for all the games on the VC (save maybe the Genesis games, but that original controller was wonky to begin with), and frankly we design controllers better now than we did in the 80s and 90s as we've taken into account ergonomics and allowing for a wide variety of experiences. I saw with the original Classic Controller that I don't like using controllers that don't have grip handles anymore, for instance.