Here's the deal. I own a Wii, have several games, but truth be told, WiiSports is good enough for most folks. Look at the Wii numbers - Mario sells, WiiPlay sells, but that's really it. Developers are down to make loot. And using last-gen technology to make games is easier than using multi-million-dollar budgets for games that the Wii crowd *may* not buy. Look at MP3. It was doing good, but not awesome. And that's a 1st party title!
Even as popular as the Wii is, it is really a risky venture for developers. If it's not Mario or offer a free remote, it's the luck of the draw.