Why? It’s free for open source, reasonable for enterprise and private use and I think you could build a better one starting from scratch. GitHub’s success is largely due to the network effect and it’s entrenched status as the canonical code repository. Besides libgit2, aka “the secret sauce”, is already open source. What are you waiting for?
Github has had stunning stability. I bet their architecture is nontrivial, that dozens of parts of their code are tied intricately to their architecture, and that opensourcing only the bits that will run on typical AMIs or on debian^Wdevuan would be little more than opensourcing helloworld.rb.
Has it really ? I remember many threads here on complaining about Github's up time
https://news.ycombinator.com/item?id=5808496