Github's workflow, features and terminology are focused on open source projects and those projects tend to work in a specific way. Kiln is focused on providing features that fit better inside a business environment. For example, the idea of a pull request is a bit foreign in a business environment. Internally, almost all the code written eventually gets shipped, so the social interaction around pull requests doesn't fit exactly right.
Joel explains it better than I can: http://www.joelonsoftware.com/items/2013/03/11.html