Thank you. It seems to be a case of "use it or lose it". Our ancestors diet came to include sufficient fruit so that there was no selection pressure to maintain genes for vitamin C.
The specific mutation can be identified and usefully confirms our relationship to other primates. A significant part of our genome consists of pseudo genes. Relics of our past. See the thread in this series on olfactory genes for another example. Typing on my phone so can't do a link.