This is the reason.
The PS3 launch price in the US was $499 to $599.
The XBox360 launch price - and the price for several years - was lower than the PS3. By the time they were similar in price, it was too late to make up significant ground. The PS3 still does not have a $199 model on the market.
Looks like it's time for a price cut on the 4GB Model.
Michael Patchner stated in an episode of Bonus Round on GT in 2010 that Arcade 360's account for less then 20% of 360 sales. I see no reason for this pattern to change with the introduction of a 4GB model instead. If anything people are paying more for 360's because of Kinect Bundles.
Also while PS3 may not have a $199 model, it wasn't hard to find a $199 model in the USA during the holiday season. Or at least a $250 model bundled with Uncharted 3 and InFamous 2.