Advertising for a game can only do so much to draw in new players, but retaining the old players is half the battle. In order to do that games must create expansions that keep the content live and interesting. The average player sticks with a game to completion, and each game is different. Once an average 'time of completion' has been established, gaming companies should use this to time the release of expansions. For most games, this would likely be within a year, but very seldom over that limit. Too much time and the player loses interest in doing the same old things in the old content.
Much like a series would release in movie form one year to more apart, game expansions are the same. If a game expansion is released to close together then those playing the game do not have time to enjoy, learn or become experts at one expansion before another is released. This results in overlapping and would likely result in a quicker decline of interest.
Game coding and graphics are very complicated. One game could take years to make before going into a beta state. If more pressure to put expansions or sequels out faster is put on the developers, it wouldn't be as good. They need to be able to take their time and make it right. I think their deadlines are already crazy enough as it is.
Games are expensive and I think people would be more concerned with whether or not they are getting their money's worth than with how fast expansions are released. If the original game is good enough people may be able to stay occupied with it until the expansion comes out anyway, and will be looking forward to its arrival. In retail, anticipation can be a very good thing. A year isn't a very long time to wait.