Changing it could mean they made the wrong decision in the first place, which could make Important People lose face by possibly being at fault. Of course, they could try to present things in a different way, but potentially inviting criticism by rectifying mistakes isn't going to be looked upon warmly unless the people involved are all dead or out of favor.

It's also not exclusive to China/authoritarian countries. Plenty of European countries have de-facto decriminalized cannabis by rarely enforcing laws on possession (I'm reminded of how often I saw people openly smoking weed on the street in Berlin years ago), yet politicians don't want to be the one to stick their neck out and say that the war on drugs failed or cosign the use of a particular substance -- even if their electorate largely would agree with them.

It would be an embarrassment if the reason it outlived its usefulness is because it got it got circumvented by hostile actors.