The question is about developer support.
The Wii could have sold 200 million, 300 million ... if the fact that 95% of the development community ignores the Wii is still constant, then what's the point? It's great that they were able to sell to soccer moms, but that doesn't change the fact that BioShock and Mass Effect and Skyrim and Battlefield 3, etc. etc. etc. are not on Wii.
A proper market leader, like the NES, SNES, PS, and PS2 have the majority of the backing of the development community and has the most active software ecosystem for the majority of its life cycle. The Wii by year 5 is the same story by year 5 as the N64 and GameCube were -- a couple of Nintendo games to run out the stretch and basically next to no really interesting content from any other developer. It's a one man show.
Like I look at this way ... what's the point of having the biggest venue for a party if you only have 5 people still left by 12 AM? And the guy down the street in a smaller venue has 200 people still there partying it up well past 3 AM.
It's great that Nintendo was able to expand the market to soccer moms and non-traditional demographics, but these audiences haven't expanded the reach of Nintendo platforms to audiences that actually buy games outside of the party/dance/mini-game compilations zone (and worse, you now get that same kind of experience on the XBox 360 or PS3 if you really need something like that ... Wii Sports was all the rage in 2006 ... today that type of game has been copied and played out over and over again that it's not interesting anymore).