Look at the chart above and compare SCEA-published titles to the exclusives for other consoles like Halo 3 (which sold more than 4.8 million copies in its first year) and Mario Kart Wii (which has sold over 6 million units). Cleary, SCEA isn't anywhere near Nintendo's or Microsoft's numbers (all figures excluding bundle sales), but why can't SCEA games achieve that sort of level of sales? GSW points out it's mainly because of Sony's focus on risky new IP instead of safe bets (like sequels), noting third-party franchises like Call of Duty: World at War (1.1 million), Grand Theft Auto IV (1.7 million) and Metal Gear Solid 4 (1.1 million) have all sold better.
It's all a rather interesting interpretation of the NPD sales data, and there's even further analysis to be read over at GameSetWatch. Head over there for the full read.