The costs of setting up a geographically redundant GitHub clone and continuously updating software and configuration are far in excess of the mere $200/month GitHub charges for their Platinum plan: unlimited users, unlimited public repositories and 125 private repositories. Even the Bronze plan for $25/month with 10 private repositories is exceptional value to businesses that only develop a few pieces of software.
At GitHub's price points, a manager would be crazy to take on the risk, expense and effort of setting up their own clone. The obvious "but..." excuses don't even apply because:
1. GitHub offers enterprise "run GitHub from your own server room" services for companies that have greater needs for confidentiality/customisability.
2. Private organisations can take their source code and other data with them. This means that users are free to move elsewhere (regardless of whether GitHub is entirely open sourced) if GitHub suddenly decides to charge $1,000/month/user.
And how many businesses really need support for geographically redundant GitHub clones with continuously updating software and configuration ?
If anything, them having this option at all makes no sense, as the real value of GitHub is in the collaboration and discovery tools it brings, useful for contributors working remotely.
Unlimited users, unlimited public repositories -- I have that on my own VPS. I just configured my own Git repository. It's a piece of cake. Even companies like Adobe get along just fine with a Wiki and a Perforce repository.
Basically when selling a product or a service, you do have to ask yourself: how big is the market for this? And I'm not seeing many companies really needing private installs of GitHub and that market is even smaller if the product would be open-source.
Something like RHEL, the support is worth it to many businesses.
Something like Github... I could see it being less of a priority once it is up and running.