It's usually just because they have to pay an on-going licence fee to keep using it, and after a certain amount of time, the revenue that a particular game is expected to bring in isn't worth the fee.
Lots of studios use Denovo to try to reduce piracy during the release period of the game, and then remove it when the impact of piracy would be lower. It probably involves everything from managing customers good will to the cost of Denovo over time.