I know nothing about the legal profession, but my reaction would be because wiki software sucks. I'm a designer and a developer and I can barely figure out how to edit a Wikipedia page.
Another question (which I don't know the answer to) is how many developers had never used version control until they were introduced to Github?
I bet there are some seriously big interaction improvements you can make to the average enterprise "collaboration" software.
It wouldn't take that much work to change markup languages, or fix the UI of something like mediawiki or dokuwiki to something that's more amenable to those industries, if they don't like the defaults. Mediawiki might be pretty locked into a database for page storage, but dokuwiki has a git backend plugin, and some other wiki software even supports git backends natively.
Another question (which I don't know the answer to) is how many developers had never used version control until they were introduced to Github?
I bet there are some seriously big interaction improvements you can make to the average enterprise "collaboration" software.