Its about focus, which I believe is essential for both great products and successful startups. By focussing on GitHub only (we also focus pretty much on web apps running on Linux) we are able to make a great experience with limited resources. If we had said we'll also support svn and objc and windows and iphone and embedded systems, we'd be all over the place.
I consider it like Heroku vs Amazon. You can do anything with Amazon (like on Jenkins), but with Heroku it just works (like on Circle).
I don't get it, heroku is not a github plugin, I don't see how providing a clone url on a different domain would make you unfocused. But it's your business, so good luck anyway.
The Status API, how we clone quickly, the entire signup workflow are all built around Github. We have special logic for github downtime, retries, handling 3rd party repos, collaborators, etc. For example, we dont have usernames or passwords or password resets for Circle, we just have github oauth. We don't have teams, we just mirror the github permission model. We don't verify your email: github does it.
Most importantly, when you sign up, there's a great flow. You give us oauth permission, and pick a repo (we have a list because Github) and then we can have you running tests in seconds. We automatically fiddle with Github to make that work (you dont need to add post-commit hooks or ssh keys, it just works).
I consider it like Heroku vs Amazon. You can do anything with Amazon (like on Jenkins), but with Heroku it just works (like on Circle).