Maybe that's a start? Techies would find it easier to promote the use of proper VCS for legal documents if their products used them for their ToS, and exposed them to users, e.g. in form of diffs. There's even a precedent - Wikipedia uses such interface for all their pages, including their ToS - see https://foundation.wikimedia.org/w/index.php?title=Terms_of_....

Beyond technical hurdles that could be solved with a set of FLOSS components and plugins to popular CMSes, I don't see any reason not to do this if one runs a honest company.