Well I haven't personally seen it, you'd be better off telling someone at Telltale, but there could be many reasons. I don't know off the top of my head how much space the PC version of Season 1 takes up and whether or not it's on a regular or dual layer DVD, but the Wii would be using the same amount of space.
Maybe the way the engine on Wii interacts lead to an increase in audio or video size, and therefore an easy way to save space is to resave all the textures in a lower resolution. Again I can't confirm whether the textures it looks really blurry or blocky since I haven't seen it on Wii, and the difference could just come from seeing it on a bigger screen. I don't know. They may have not resaved or resampled the textures at all.
I don't work there, so I can't offer you a good answer. If you complain on the Telltale board, even though that's sort of tacky, they can't fix your game but they can probably give you a good reason.