Despite the many replies in this topic, it seems that we have only scratched the surface of the business argument in favor of keeping God of War and Yakuza on the PS2. Tell me, what do God of War, Yakuza, and Shenmue have in common? The answer: It's the initial cost of development.
(Granted one could easily make an argument involving the development costs of Shenmue and Yakuza, while God of War's development costs are a bit more of a mystery. But just looking at God of War could tell you that it didn't have a small development price tag.)
And what do you when development costs for the first game are high? Answer: You make a sequel using the same engine, with some minor, yet noticeable, enhancements. In nearly every console game development process, the majority of the costs will most likely go towards building the engine behind the game. Once you have the engine, you can churn out sequels at a lower cost using that engine. Numerous companies do this practice, even if the original game had low costs and high profits, but high costs can encourage this practice further.
(I'm thinking about Sega's past plans for Shenmue...)
There are two problems with moving God of War and Yakuza to the PS3:
1. The developers would have to deal with creating a new engine to suit the PS3's different architecture and capabilities.
2. (Mostly applies to God of War) When you have a successful franchise like God of War, which showed consumers what the PS2 was capable of graphically, expectations are much, much higher, and the pressure is much, much greater if the developers decide to release the sequel on a next-gen console. There is the potential of the franchise losing credibility if the next-gen sequel doesn't feel as revolutionary or important as the first. At least with the PS2 release, not only are you reaching a larger fan base, these fans will most likely have lower expectations, as the sequel will still be considered an impressive feat for the console.
(Phew! I was going to talk about something else, but I forget what it was...)