Maintaining and improving one of the largest websites in the world using Open Source software requires a continuous commitment. The site is always evolving, so for every new component we want (or need!) to deploy, we need to evaluate the Open Source solutions available.
One of the main aims of Wikidata is to represent knowledge in a way that is computable—that is, amenable to automatic processing. Wikipedia already contains a lot of information; much of it is reasonably easy for a human to understand—though some of the more esoteric bits are decidedly not—but it’s not at all readily crunchable by a computer.