If I were to guess, this is probably to help them move off of Rackspace onto their own hosting solution in order to appeal to that enterprise customer segment that keeps getting mentioned.
But GitHub's enterprise application is something enterprises can host themselves, inside their firewall. https://enterprise.github.com/
I'm referring to the use-case where it's something that's truly cloud-based–think more like Salesforce. Most enterprises are (slowly) shifting to this.