Despite recent claims that Nintendo is "lazy," I believe that, basically, whenever Nintendo sets about to make a game, they have a sincere interest in making it as best as they can. Doesn't mean that they always succeed--Nintendo can fail, too--but I think they make strong efforts at creating "quality" games for their systems.
It would be difficult for me to judge the efforts of 3rd parties; I don't work for them. However, looking at the results, I find it really hard to believe that they're putting in the same kind of effort into their Wii games as Nintendo does. And that's what sucks.
It's not about "investment" as money, but overall investment: time, thought, concern, etc. Sure, a masterfully produced Wii game will, on average, cost less than a masterfully produced HD game. That's not a problem. In fact, that's great for the publisher. But sadly, I think that some of these third parties have taken the idea that Wii games cost less to mean that they can half-ass an effort, and it'll still be okay.
Over the years, Nintendo has built up a rep for good games. I think it's easier for a Nintendo game to reach, say, 10 million sales, than a game from another given company. But I also think that those kinds of numbers *aren't* unreachable for a third party on the Wii, provided that it's the right game.
What would a third party have to do, in terms of investment, to achieve a game of Nintendo quality? And how much would they have to market such a game, to eclipse 5, 10, 15 million in sales, or more? I don't know. But it's a shame--for them and for us--that they don't seem to have an interest in doing it. Or in trying, really.