Previous/Next is a bit complicated if one of the key features of the versioning system you're using is that it's a graph.

Of course, that graph could easily be flattened (e.g. simply sort by date) but that'd cause you to jump all over the graph. It would make for a terrible and confusing user experience as the next commit is suddenly based on a state of the code you saw 20 steps ago.

As it would only work properly for projects with a completely linear history (which most projects don't have) I can understand Github not adding this feature. I'd say this is something a third party app using the Github API could do very well for projects specifically set up for learning through their history (like this one).