Following release, a lot of developers add extra content to maintain the interest of players. This can be anything from paid expansion packs, to free updates. As with anything, these typically take a while to put together, except stupid Day 1 DLC.
Those who purchase the game on launch and finish it pretty quickly have to wait several months before they can get hold of an expansion that they have been dying to play. Whereas those who wait 6 months to a year can pick up the extra content from the get go, and jump into it straight away.
The Sims franchise is a fantastic example of this. The Sims 2 and The Sims 3 took four years to reach the amount of content they now boast, meaning the players who waited until the final expansion could play the titles in their entirety. However those who have recently purchased The Sims 4 will spend the next few years being drip-fed with new expansion packs every few months.