Its a combination of problems...and a vicious cycle
To start this generation very few developers (almost none) were betting that the Wii will become that succesful, so it took them by surprise and they were already developing their new franchises for the other machines. To try to get some money rom the system, they tried to produce some very poor quality games very fast to sell them to the new players, a lot of those were sold, but most of the buyers got burn by the lack of third party quality games, unlike the Nintendo products that were always top notch.
Now when there is a good third party game in development only the few minority of gamers that follow the gaming news actually buy them, and that's not enought to keep making big budget developments for the console, also the multiple console owners even if they are interested in a few Wii games, only have a limited budget and they usually use it to buy games in the other consoles, because they know the franchises or trust the effort of the developers for the other consoles.
