Sometimes new code creates and persists state that the old code can't handle and rolling back would just make the old version blow up because of it. Or worse,

Sometimes new code creates and persists state that the old code can't handle and rolling back would just make the old version blow up because of it. Or worse, the new feature involved the tracking of something in the real world, such as inventory or money, and rolling back after the feature had been used by a real customer would lose that. The number of times I’ve had to explain to webdevs that when a customer asks where their money is “in a temp table” is not an acceptable answer.