What? There's tons of third party support. At almost every given moment of this generation, there has been more third party games for Wii than for any other system on the market.
Let's just cut to the real question. What you're really asking is why the Wii doesn't have the next big installments of Resident Evil, GTA, JRPG XIII, Platinum Games Presents, Castlevania Of War, or whatever the hell else it is that's igniting the latest raging message-board megathreads, pixel-counting screenshot wars, and fawning, five-star video reviews.
Thing is, by the time you can sit through a thirty minute cutscene about a gender-bending ninja cyborg limping around to save a mustached, one-eyed super-soldier, you probably also cut pretty deep into the same demographic that spends large quantities of cash on electronic entertainment, buys way more HDMI cables than the per-capita average, and scorns the suggestion of physical exercise encroaching upon their grossly disproportionate interest in recreational software.
What I'm driving at is that if you were to create a ven diagram that mapped out the type of people who are actually interested in the specific types of product that are absent, I'd be willing to bet that you'd find a nearly perfect and complete overlap with the type of people who are interested in better graphics, HDTVs, realistic physics, and online play.
Nintendo's biggest success was identifying that the presumed critical mass of spaceship fantasizing, underboob-ogling, IGN-reading geeks were actually a fractional niche.
It seems pretty clear that this niche is now struggling to catch up with a moment of self-realization.
Hate it, bite it, fight it.....the answer is still, quite simply, that despite the Wii's resounding success, it has not managed, on any level, to corner the increasingly irrelevant traditional market that continually slaps $60 across all of those sweat-smeared Gamestop counters to buy this type of nerdy crap. Those folks run with a different crew.
And without them, you really can't sell Street Fighter.
Oh, I know. You, or I, or Steve, or poster #213, or whichever petition-posting collection of 2,000 people all have Wii's and we like them, and we'd buy 3rd party hardcore games, and blah blah blah whatever.
When it comes to gaming the 'core gamer-market game, the Wii is basically stuck playing the Spock goatee-wearing, inverse-world version of Kinect.
A day late and a dollar short. Just the way it is.