When a file is moved or renamed, the GitHub History doesn't show commits from before the move. Git itself supports the --follow option to show full history in these cases (and apparently it's much less buggy now then it used to be), but there is no way to achieve this in GitHub.

This has been requested many times, but has yet to be implemented.

Current workaround seems to be installing this Chrome extension, but that's insufficient for a large dev team.