Well, the "why" is certainly "because you use git from the command line; no context switch; CLI is always faster then GUI".
And of course these reasons are poor and hide the fact that with systems like Github (Gitlab and Bitbucket are the same), issue tracking and thelike are locked in. I only know the wiki which can be accessed as git repository (at least in Gitlab and Github). Which other Github "collections" are exposed as git repository?