Github is a very nice service in many ways, but it is a proprietary service, running (at least partly) on non-free software. Obnam is free software, primarily developed by a free software developer, and hosting it on a proprietary service would be unethical.
Services such as Github often provide many things on top of mere code hosting, such as ticketing systems. The services might be very easy to use. It is true that Obnam, which is self-hosted by its main developer, could benefit from some of those things. None of this takes away the ethical dilemma of using proprietary software.
For the difference between open source and free software, see for example Software freedom: an introduction.