Maybe because it was tempting: JSON is fairly easy to handle, very portable, and when you look at a JSON document, it's straightforward to think about querying it, and thus DB, although JSON is structured, and DBs are relational.
> although JSON is structured, and DBs are relational.
You can have structured relational databases (RethinkDB for one, but there are others)
Job security, maybe? Can't think of anything else... This person would certainly never be fireable again after merging that monstrosity