Most games on the most popular system (check the PS2, it'll have similar traits) are utter tosh and not worth anyone's time, so they drag down the average.
All platforms have bad games, but the most popular has the most bad games since it has the most games in total, so the statistics stand out a bit more.
Nothing fancy to it, really.
While there are probably almost as many 360 games as Wii games, it could be argued that most 3rd party Wii games have been... lacking.
When there was just the 360 in the market, dev's had to concerntrate a bit, bad games wouldn't drown in the flow of other games, they'd rather get attention, attention of the bad kind.
This is always the same when a console becomes a hit, the sheer number of so called shovelware is almost to huge to fathom.







