Why do teachers grade a b c d f instead of e?

The grading system of A, B, C, D, and F is a historical convention. There's no single definitive reason why "E" isn't used, but the most likely explanation is that it was simply omitted from the established system. The letters likely corresponded to different levels of achievement with a "failing" grade needing a distinct marker (F) separate from the passing grades. The use of letters themselves might have been chosen for brevity and ease of recording. There's no inherent logic that dictates the specific letters used, other than cultural acceptance.