So what's your conclusion from this failure? That everyone else is stupid? You're a bad teacher? Or it's not actually simple and the above explanation includes a ton of implicit understanding of the subtle interactions of the various moving parts?
For the majority of programmers I think it's lack of experience with data structures of any kind. C programmers have to understand pointers and most (I assume) would have implemented at the very least their own linked list at some point and maybe even a tree. But there are so many programmers who simply lack this experience so talk of pointers, links, graphs etc. is unfamiliar.
Then there are those whom I'm sure should have the necessary experience (because they are C programmers, for example), but still don't seem to get it. These people I think just don't care. They don't care about version control and therefore it's irrelevant what git is trying to represent. They just want to get their code merged.
That reminds me about the old joke about monads: At the moment you finally understand them, you lose the ability to explain them.
Seriously, I too find the basic concepts of git quite simple. But whenever I want to do anything slightly out of the ordinary, I find myself wasting a lot of time searching the docs. In fact, I find the naming of commands and their options almost the opposite of intuitive, given my understanding of the basic model.
I can't use the command line at all. It's horrendous and makes no sense. I use magit for everything if I can. If I can't then, like you, I have to spend ages searching the docs.
The way I tried to understand Git at first was like Subversion. Horrible. I almost deleted everything my team worked on for weeks.
Then I read "git inside out" [1] (not to be confused by "git from the bottom up" which I think is not as good), had a "aha!" moment, my view changed and everything became clear and easy. Transformation from graph to graph is something I do every day, so why not in Git?