Back in the era of NES and Genesis, it wasn't unheard of for sequels to release a year later. It's just over time, games have become more complex requiring longer development times. Today's average gamer is used to some title they loved having a sequel two or three years down the line, due to the development time of that sequel.
However, companies are putting more time into developing game engine that will last, allowing for sequels to be producted faster. With a tried and true game engine, the next game only requires some new levels and characters to make a sequel. Granted, some games like Half Life 2 Episode 3, which has a lot of levels, cut scenes, and voice acting is probably the biggest reason it's taking so damn long, but something like L4D2 where you make a map and the game engine generates enemies and the voice acting is sparce allow for a quicker turn-around on sequels.