I think they all had lower budgets than big-name hits, lower advertising budgets, and most were made as afterthoughts in an attempt to cash in on the most successful current platform.
Those that weren't afterthoughts, Mad World, House of the Dead: Overkill, and No More Heroes, have all been successes, considering Sega is thinking about a Mad World Sequel, said they were happy with HotD:O, and a sequel to No More Heroes 2 came out today.
Name another poorly-selling Mature game that isn't a cash in or a fiscal success, and maybe we'll start talking. Still, remember games like GTA 4, and Modern Warfare 2 have gigantic budgets, both in development and marketing. Any game given that type of treatment on the Wii has done well. The problem? Nintendo is the only company giving games that type of treatment on the Wii.
There you go, I think most of us here agree on something more or less similar to that. Go dig up a talkback thread about Deadspace if you want more.