I hate to be the one disagreeing with everyone, but I do think Nintendo shoulders some of the blame. If a developer spends time and resources on a title, they could produce a title of Res Evil 4 quality......but look at RE4. That game was in development forever, and pushed the Gamecube to the limit. Companies can work hard producing a game that would push the Wii (The Conduit, for example) and it would still not be up to HD standards. Trying to tap the Wii's unseen power to make a game that's great buy last gen's standards is a huge risk.
Like it or not, having hundreds of enemies onscreen in Kameo or Resistance 2, impressive character models like Uncharted or Metal Gear 4, having bosses the size of houses in Bulletwitch or Gears of War 2, having amazing atmosphere in Dead Space or Condemned 2, highly destructable environments in Mercenaries 2, or huge sprawling environments in Halo 3 or Fable 2 make the games better. Our standards have risen. When I drove a used Buick Sentry, it was great at the time. I drive a better car now. 6th gen quality games were great during the 6th gen. Better experiences are expected now. It's depressing that, not even Nintendo's own titles are much better than Zelda TP -a game that was fully realized on 2001's Gamecube.
I love Nintendo's games, but imagine a Zelda game with Fable 2's size and scope. Imagine a Metroid with Gears 2's graphics. Imagine Mario Kart with "Xbox Live" like online. Imagine Mario Galaxy.....well, that game was perfect.
If I were a designer, and I were going to spend years on a game, personally, I'd do it on a console that could fulfill my vision. The Wii is fun, and I do love Nintendo, but the Wii just isn't that system.