Developing companies take the Wii seriously, but porting demanding PS3/360 games to the Wii or PS2 is a lot more difficult than just making the game run in a lower output resolution. The Wii has a much less processing power and memory meaning a game must be dumbed down in other areas as well.
Then you have the problem of many demanding games being primarily targetted at late-teen and adult males, such people are likely to own a PS3, 360 or PC as well and for games like BioShock or Oblivion IMO it just makes more sense to buy a version for a better specced system instead.







