There's two arguments I can think of for this.

Among whatever other reasons for sentencing someone, there's an almost circular logic element of punishing people for breaking the law. In modern society there's an unspoken agreement that people will follow the law, even if they don't agree with it. Therefor, in essence, breaking the law itself is breaking the law, which doesn't change even if the act they did later becomes legal.

Second is the practical element of a clogged "justice" system. It might be simple to release a person who was convicted of a newly legal act, but it still requires review. Plus what about other cases, where they might have been convicted of multiple counts of different crimes? You'd need to resentence them since it may not be clear how much of the total sentence was derived from each crime. What about people who were only fined? Under the same logic, wouldn't they deserve their fines back? And how far back would you go with these repayments?

And on another practical level, if a change in law results in "criminals" being released and large repayments, voters that would be supportive, or at least apathetic, may come to oppose it.