Being that we know the Wii is more powerful than the Gamecube and few Wii games look as good as good looking Gamecube games, I think it is safe to assume that developers are not really focused on pushing the Wii's hardware to its limits ... I could be wrong but I suspect the focus on pushing hardware on other consoles comes from the developer's assumption that people bought that system because of how powerful it was, and they came to that conclusion because every console has been sold to consumers based on its processing power. I could be wrong but I suspect that few developers see pushing the Wii's hardware as being a necessity to selling games, so it becomes a very low priority.
Personally, I agree with them and as long as the visuals are well polished and the art direction is decent there isn't much of a need to push the Wii's hardware to sell games.