Becasue you have to have AAA games to make that price worth it. Sure the Wii doesn't have the greatest library right now, but they don't need it as badly because the $250 price gives them some slack.
But if the PS3 gets the games it will do well. Games are what make all the diffference. For me the $500/600 price tag isn't too high, but there has to be a great reason to pay that money. Show me the games, I show you the money.