It's not install base (Wii) and it's not user base either. The only factor in third party releases is how well their games will sell on a given platform. If they feel their title won't be worth the effort to port it or bring it to a console, they won't do it and that's been Nintendo's problem since the middle/end of the SNES era.
When SOA and their Genesis Does campaign overtook them in the US for a few years, their image with teenagers and young adults were damaged forever. They tried to rectify with Gamecube but failed.
Until we see a day when the average Madden and COD guys buy Nintendo consoles, we're going to continue to see problems with third parties. Even games that don't sell to that particular demographic will stay away because as the Madden/COD gamers go, so too goes the mainstream consumer and that's needed for third party games to sell at volume.







