Although San Andreas was a great game, it was missing a lot of things (A well-populated world, good character models, to name a few)... Rockstar had already hit the tech ceiling on the PS2, and you could tell. There is no way that the next GTA would be made for PS2, the nature of the game forbids it.
Someone said it best; the "Stories" releases that they have been doing are good enough for last-gen.
Besides, GTA doesn't need to speak to the broad audience. It is a system seller (not a parcular system, but systems in general). That's why you still see stuff like license-based games come out on last-gen consoles too. "Spiderman" isn't being aimed at hardcore gamers, it is being aimed at millions of Spiderman fans who happen to own a game console.
GTA is aimed at gamers specifically. Since there are no other tie-ins with the IP (no movies, comics, shows, collectible cards, bubblegum, whatever), in order for someone to like GTA, he/she need to like video games. And if you like video games that much, you will find a way to buy a PS3 or a 360. Whereas, people who might think of trying the Spiderman game might not be hardcore gamers at all. So THAT is the type of game you dont want to "ignore" a large install base, especially so early in the transition period.
Of course, releasing GTA4 for PS2 will sell tons of units. But all that means is that millions of people who buy the game on PS2 wont be buying it for PS3/360. And that means that they just spent millions of dollars "dumbing down" a game to last-gen, so that gamers dont HAVE to buy the version that Rockstar spent TENS of millions of dollars on originally.
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