There are a lot of Super, Special, Stylish, Superlative, Stupendous, Spectacular words that begin with "S". Some games have a Star instead, further justifying it.
Also, Devil May Cry 4 (and other games in the series) spells out what the ranks stand for, providing some examples. Deadly, Carnage, Brutal, Atomic, Stylish, Smokin' Style, Smokin' Sick Style.
Sugoi, Japanese meaning "amazing" or "wonderful", etc.
Why is there ranke E? F is for failing, but at least you passed. Should there be a D?
If you're asking why a "traditional" American-style scale (A B C D F) lacks an E, my best guess is that the folks who originally came up with it A) wanted F for Failed for obvious reasons, B) wanted each letter to be its own 10% block, for simplicity, and C) decided the pass-fail boundary should be at 60% for whatever reason. Deciding between fulfilling their requirements and including an E for completeness's sake, they opted out of it. But (while it makes sense) some other derivation could be the right one..
A still occasionally used and almost certainly older grading scale had "E" for "Excellent/Exceeds Expectations" as the highest grade. Presumably, schools didn't want kids to have confusing grades (among other things, lying about which scale it is when they get an E. When I was graded on this scale in elementary school, it ended in an "F" for "Failing", so that wasn't a problem).
My middle school used that, at least a while ago, for conduct. The system went "E", "G", "S", "N" "U". That's Excellent, Good, Satisfactory, Need's improvement, and Unsatisfactory, respectively.
Schools outside the U.S. have E as a fail rank, where D is the minimum pass.
This is also sort of why a lot of elementary schools have switched to either symbols (checks, pluses, and minuses, where plus is best, check is middling and minus is worst) or numbers (often 1 to 4 or 1 to 5, with either end being the best or worst) for grades. When I was in elementary school, I would usually get numbers for report cards and symbols on homework.