They think these "adult" games wouldn't perform on the Wii. Not the case imo. I'll bet that if they developed a GTA game specifically for the Wii i.e. not a shoddy port but a an all-new exclusive title, and pushed it with any sort of advertising like they did with GTAIV, it'd sell extremely well. Extremely.
The simple fact is there hasn't really been a high-profile Wii-centric "mature" title released and pushed like that. We've seen a few ports such as RE4 (which did manage to sell well) but that's it.