As evidenced by homebrew-only games on the Wii, to bring out the full potential in a console the developers have to actually know wtf they’re doing and care enough to sit there and do it. When you’re trying to port a game from one system to another, the care just isn’t there, so whatever console it was originally developed for is going to have the better looking graphics.
MS went WAY out of their way to make sure the XBOX360 was easy to program games for, as did Nintendo, Sony thought that since their hardware was better developers would automatically want to re-learn everything just to program for the PS3. Well, surprise surprise, that didn’t happen, not quickly anyway.