It ties in nicely with the other open source (compatible) solutions they're trying to make money off of right now. The money is in GitHub Enterprise, and in how

It ties in nicely with the other open source (compatible) solutions they're trying to make money off of right now. The money is in GitHub Enterprise, and in how they can roll the product's workflow, issue management, and huge developer base seamlessly into their cloud offerings. They don't lose any customers by open sourcing it. Nobody's going to use GitLab if they can just download GitHub, and companies will still pay for Enterprise because they still need support for their massive investment in GitHub's extended features.