Probably because a lot of people made the same assumption (and mistake) that Nintendo did - that userbase would translate into support. It didn't, for a number of reasons.
The Wii's strongest years were early on when they delivered a succession of massively popular titles - Wii Sports, Wii Fit, MK Wii etc. - and blew their load in terms of supporting software - Twilight Princess, SMG, Super Paper Mario etc.
When they entered what seems to be a traditional Nintendo second/third-year lull, there was little there to offer the more "core" audience, and by the time Nintendo's next wave of software and more serious third-party efforts arrived it was too late.
Of course, there were other factors, but I stand by my belief that if the wave of support that hit in late-2008 and early-2009 had arrived a year earlier, we would be having a very different discussion today.