Grading system: Why A-F and not E? Why do we use F for failing on an otherwise arbitrary alphabetic grading scale? Isn't E just as meaningful for failing as A is for excellent?

I'm more interested in a response on the history of how the current scale came to be.

(Edit: prompted from the comments below, I am asking about this situation within the context of the American grading system.)
I think, and this is really only conjecture since I don't know the history, is the potential conflict with the ESNU system where E stands for excellent. The ESNU was (and potentially still is) used in US based elementary education.