Making a complex system like this available for general use cases, especially when it's still under active feature development, is hard. It also commits you to providing documentation, upgrade paths, incorporating changes that aren't useful to you and may also cause problems for you, and just generally complicating development and maintenance immensely to the point of drastically slowing development.
My guess is when it's stable or they start building their next version control system, they will start opening it up.
I know especially in this day and age when developing from scratch is so easy, I write most software for myself first and it's way faster to develop than trying to build shared bits. That isn't great for the future of software, however the tragedy of the commons isn't anyone's problem, by definition.