Because it's an extension of GTAIV. It's not a whole new location, with a whole new story line with another 100 or so missions. At least; I think it's not.
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I believe people did it in 9 hours from the rockstar club thingy iirc
I thought that the storyline was pretty good, a good length for a game too.
Completed it for the first time back in April in 29hrs then completed it a few months ago in 18.
I did it in 19 hours :)
But straight missions, no hanging with friends, or messing about.
I Got It! :D
The Guns Are Really Hard To Control Using The Touch-Screen! :D