One of the most frequent questions in a game (OT, Review, etc) thread is "Do I need to play the first game before play the sequel?"... so some gamers just enters in the franchise if they can buy the previous games at cheaper prices.
I did that with a lot of games: MGSIV, Mass Effect 3, Borderlands 2, etc... some games I even bought the sequel together with the previous cheaper game. That is my point... cheaper games near the launch of a sequel can make more gamers enter in the franchise.
I'm sure a lot of gamers brought Uncharted to play before the sequel after see all these 100 reviews of Uncharted 2.