That's not why they did it, or at least not the only reason. They held it back to polish it a little more!
Sort of a shame that in pictures it looks like a toned down version of Half Life 2.
But you know how it is.. GTA games have never been great for eye-candy. Plus, the draw distance has always been far for scenery.. but awful for pedestrians and cars. Evil, evil popup/fade-in issues in every last GTA ever made so far.
Then again, this definitely looks better than San Andreas, VC and LC stories.