"K" is also used in baseball
-speak to mean strikeout
Why "K" and not "S"? Most baseball historians (and baseball websites I've browsed) seem to agree the answer comes from the very early days of baseball, back in the 1850s.
A sportswriter named Henry Chadwick created a method to keep score during a game, so he'd have something to reference whenever he wrote articles. This scoring system is the basis of modern ones that can be found in scorecards bought at baseball games.
So, why "K"? Chadwick used "S" for "sacrifice". So he used "K", which was the last letter as "struck", a then-common term for a strikeout. Makes sense.
Dwight "Doc" Gooden became known as Doctor K in his heyday as a member of the New York Mets in the '80s. Fans in an area of Shea Stadium that became known as the "K Corner" would hang placards with the letter "K" for every strikeout Gooden notched that game. This tradition has been used for other power pitchers since.