The Nintendo brand name power argument, besides being simplistic, is also ignorant of the fact that the mainstream can't often tell the difference between brands. Everything on the Wii is a Nintendo Game or a Wii Game as far as many mainstream customers are concerned. They do recognize trademark Nintendo characters such as Mario for example. But it's not that the mainstream is stupid. The mainstream user is /not/ a hardcore gamer fanboy who cares what company published what game, who's making the most money, what game sells more units etc. But the reason why nobody picked up something like ExciteBots is because, surprise surprise, it looked stupid and unappealing. Even with "Nintendo" on the box.
Most 3rd party releases on the Wii actually suffer from assuming that the customer is a moron. Wii game packaging, covers, presentation, are cheap and insulting. (Even Nintendo gets this wrong sometimes.) Even on some good games, they're not presented in an appealing way. Silent Hill: Shattered Memories' crappy photoshop cover with its cheap font looks like more Wii shovelware, some faux-mature game.
To the hardcore, people buying games (or not) for what the box art looks like is an old joke used to make fun of "casual" buyers. But really, it shouldn't be a joke. For most people, video game box art /is/ the only commercial they'll ever see for a game - think of it that way. And bad boxes that make the game look trashy are negative advertisements.
Bad box art and stupid blurbs on the back that talk down to people are not the only reason 3rd parties are failing, of course; but it's just one more sign of how almost everything is being done wrong. Publishers have gotten used to having a voracious, practically captive audience with the hardcore and core gamers. (How many great games have bad presentation, bad packaging, bad commercials, but GAF members buy them anyway without a second thought?) The expanded audience is a harder sell and you have to approach them with your best foot forward while wearing a nice, clean suit, so to speak.
The music games, ironically, tend to have good presentation and packaging, good ads, good box art, etc, precisely because most music games have taken the mainstream and expanded audience seriously (because they KNOW that such games have super-wide appeal and aren't the exclusive province of niche, hardcore geeks). I also noticed that the packaging and presentation of EA Sports Active is Serious Business.
Also, about the argument that the core have /left/ the Wii after Resident Evil 4 and whatnot, there's a tiny bit of truth to that in that a lot of core gamers are so incredibly petty and fickle that yes, they sold their Wii the moment they beat Mario Galaxy because it was still a stinky casual trashbox. However, with 65 million consoles out there, PLENTY of core gamers have one. The real problem is that core gamers with a Wii bought one expecting experiences that took full advantage of what made the Wii unique.
Core gamers did not buy the Wii to play Mad World, a beat-em-up that would have been better on an HD console. Resident Evil 4 sold well because it was bloody OBVIOUS that Wii control would make a great game that was begging for pointer aiming, superior. I'd wager that most core gamers just are not excited by the kinds of "teh mature" games put on the Wii since most are experiences they could get on a 360 but with superior graphics.
... entirely aside from the fact that mature is not a genre, and an M-rating means exactly jack squat aside from your game has swear words and blood. But the industry hasn't grasped that yet in the last ten years so no reason they should now.
(Starting a countdown until someone runs in and posts a terrible cover to a Wii game that sold well anyway to prove that presentation doesn't matter. LOL?)