We don't know whether any other genes have adapted in the last 10 million years to the lack of the pathway needed to synthesize vitamin C. Reintroducing the pro

We don't know whether any other genes have adapted in the last 10 million years to the lack of the pathway needed to synthesize vitamin C. Reintroducing the proteins for this pathway may cause unintended interactions with existing proteins. Imagine a large software system that had a small mobile website. New changes to the code must be compatible with this mobile website and can't break it. Now imagine that one day this mobile website was taken out. Development continues but since there's no need to worry about the mobile site numerous incompatible changes creep in over time. Now suddenly new management comes in and decides that the mobile site should be brought back. Clearly, as soon as they try to put it back in they run into every incompatible change ever made to the code and the whole thing becomes highly unstable. So that's what (might) have happened in the body. With no worrying about whether a new mutation might negatively interact with the vitamin C pathway that new mutation can proliferate freely, especially if it is beneficial. All reverse-compatibility is lost.