Why can't you get an E on a test in the United States?
I haven't located a super reliable source, but from Slate:

The earliest record of a letter-grade system comes from Mount Holyoke College in Massachusetts in 1897. [...] The lowest grade at Mount Holyoke was an E, which represented failure. [...] One year later, administrators changed the failing grade to F and tweaked the other letters. The new scale offered better symmetry, since each grade represented five points, with scores below 75 resulting in failure. [...] Over the next two decades, variations on the letter-grade system spread across the country and into primary and secondary schools. It’s hard to put a date on the end of the E, but it was gone from most colleges by 1930. Apparently, some professors worried that students would think the grade stood for “excellent,” since F stood for “failure.”

Basically, four grades (and plus/minuses) and a fail option seemed to work out well from the instructors' perspective, and F was picked up for &quot;fail.&quot;
Possibly the concern over &quot;E&quot; was affected by the ESNU system (according to an uncited paragraph on Wikipedia):

At one time (until roughly the mid-20th century), the most popular grading system in the United States used four letters, which ranked in descending order:

E (Excellent)

S (Satisfactory)

N (needs improvement; &quot;NI&quot; was also used interchangeably)

U (unsatisfactory)