While I would love to be believe that the Wii hype train has finally crashed and common sense, like being able to clearly see a money making scheme with poor hardware and shovelware games, I think it has more to do with the fact that the Wii has reached its saturation point.
The thing has sold so many units, most people who don't spend a bit of time on game sites and keep themselves informed would have one by now, resulting in a decline in sales.