Opening a repo actually show useful stuff for me as a developer. The source is there together with an optionally inlined README file, rendered with popular micro markups. Then add the ease of forking and you clearly get something people really like. This style of presentation isn't of course for everybody. But GitHub chose to target developers above end-users.
GitHub could have been "SVNHub" or "hghub" for all I care, the technology is secondary in this case (but close behind).