The problem you're seeing is not that it sold badly for what it is on the PS3, it sold great on the Xbox 360 and that causes you to percieve a problem with the PS3 versions sales. Theres nothing wrong with the sales on the PS3 its just that the Xbox 360 had first dibs on Oblivion, they had Fallout 3, Exclusivity on Fallout 3 DLC, Fable 2 and Mass Effect 1/2. If the above games sold systems for the Xbox 360, then the gamers which bought the systems will continue to buy WRPGs for that system. The Xbox 360 has a core RPG audience and WRPGs sell up to around 3M and not more whereas the PS3 does not have as large of an audience for this time of game even relative to overall console sales.
Tease.