Why did they make call of duty 4 for ps3 and not ps2?

The PS3 console has more capability than the PS2 and the game would be better with better graphics and able to run the game program. It is much like why some computers no longer have the ability to play Call of Duty series games with the release of Call of Duty 4 Modern Warfare because it takes a more advanced computer to play the game than it did for Call off Duty 3. The PS2 had to have Call of Duty World at War Final Fronts designed specifically for them with reduced capabilities because it could not play the World at War.