The best explanation I've heard is that all it takes is a population living with environmental access to vitamin C and enough time for mutations which interfere with synthesis of it to...not do anything, and get baked into the genome, to grossly oversimplify.

You don't need any advantage for a given mutation to continue, just survival. Advantageous ones lead to new body plans, behavior, and different ecologic niches, but 'neutral' or 'negative' ones that don't get the carrier killed before reproducing are stuck with your population. If the environment makes a negative mutation irrelevant, all the more reason it won't self-regulate out of the gene pool. If most of the species ends up with it, this way, then it becomes a problem to either adapt or become extinct over when external conditions change.