Back in the NES/SNES days it was about as expensive to develop an entirely new game as it was to port an existing game due to the quantity of assembly code, the proportion of work that programmers represented at the time, and that most of the graphical assets required a ton of rework in order to be used on another platform.
The N64 and Playstation's increased development costs, increased reuseability of code and graphical assets made porting more worthwhile but few developers took advantage of it because the cost rarely justified the return.
The Gamecube, PS2 and XBox's increased development costs, increased reuseability of code and graphical assets made porting the common approach for most middle of the road properties; if you were producing a game that would not stand out on a particular platform (but also didn't need the increased exposure of exclusivity) it was important that you released the game on as many platforms as possible.
The cost of development on the PS3 and XBox 360 combined with their (at the moment) low userbases means that it is probably impossible for almost all games to make a decent profit on one platform; if you're Rockstar and you want to sell 15 Million copies of Grand Theft Auto 4 you can't release the game exclusively to either the PS3 or XBox 360 because even if everyone who owned the console bought a copy of the game you'd come up short.
