If you're playing on a Wii U, then it's the fault of the Wii U upscaling everything to 1280x720 or 1920x1080, however some TVs let you override the aspect ratio and set it to non-widescreen. If you're playing on a Wii then it's purely the fault of your TV and you need to set your TV to a non-widescreen picture mode.
Remember, just like normal Wii games, the source signal is standard definition, and there never was a standardized widescreen signal implemented for standard definition content - the "widest" signal you could get was 720x480, which is "only" a 3:2 aka 15:10 ratio (same as the GBA) which is not as wide as 16:9.
As for the stretching of 2D graphics when using the widescreen hack, most widescreen hacks work that way - that's because there isn't extra 2D content that is offscreen which can be shown - it just doesn't work that way, hence why the 2D consoles like the NES SNES do not have widescreen hacks.
Remember, just like normal Wii games, the source signal is standard definition, and there never was a standardized widescreen signal implemented for standard definition content - the "widest" signal you could get was 720x480, which is "only" a 3:2 aka 15:10 ratio (same as the GBA) which is not as wide as 16:9.
As for the stretching of 2D graphics when using the widescreen hack, most widescreen hacks work that way - that's because there isn't extra 2D content that is offscreen which can be shown - it just doesn't work that way, hence why the 2D consoles like the NES SNES do not have widescreen hacks.











