The reason is pretty simply really, youre playing a port.
In other words, they basically took a game, that on the PC runs at a max of (and I could be a little off) 1280x1024, and ported it to another system.
In this case youre playing it on the Wii, which has a max resolution of 640x480, in other words, half the resolution. Unless the graphics are hand crafted for that resolution, everythings going to look blurry and smashed, because you just took all the crap from a 10 pound crap bag, and put it in a 5 pound crap bag.
Pick your mouse back up, and save the Wii version for when your showing off next time your friends pull out their iPhones to try to show off why they think games with no buttons rock.