My question would be what Mature games should have been a success, but weren't? Whether developers choose to take the risk or not is really their own issue. It's not as though the PS3 and 360 are a safe market either. The difference there is you may not only end up with a flop, but a megaflop based on the development budget. How many Lairs, Project Gotham Racing 4s, and Strangleholds can a company withstand? I would argue not many.
I think the major issue with this generation is games are costing so much to make that every developer is restricting themselves to "Proven formulas." You ask why there are 100 FPSes on and in development for the Xbox 360, but that's exactly why. That's why Call of Duty 4 is exactly like Call of Duty 2 (Yes, I do own it). That's why Mass Effect is so similar to KOTOR. There was an interesting quote from developer Cory Balrog (GoW2 director) that said these high development costs limit creativity more than ever before.
It's not that these factors make the games bad, but it certainly means that we aren't getting very many artistic projects on the HD consoles. I miss the Okamis, Odin Spheres, and Viewtiful Joes that were so prevalent last generation thanks to the cheap development costs and high userbase, and I'm extremely happy to see that those artistic games are making it to the Wii.
Will the Wii have as many big budget games as the other 2 consoles? Absolutely not, but that's what I have a PC for. By the time developers figure out that core games like this sell on the Wii, it will be the 3rd year of the system's life (See: The Nintendo DS for reference). The Wii is trading that for niche games like JRPGs and artistic games like Zack & Wiki. I have no problems with that at all.
