And since I am here to answer your questions as they relate to this strange world inside my brain, the simple answer is: Because of C.S. Lewis.
The longer explanation is this: I don't like the look of "Sue". It's fine for other people, if that's what they want, but I don't like that extra "e". For some reason, it's always bugged me. So for years I resisted being called "Sue" or "Susie", because I was worried that someone might write either one down with an "e" on the end, and I would be stuck with it.
I was in high school when help arrived in the form of Prince Caspian. I'd read the book before, but somehow managed to skate over the children addressing Susan as "Su". But there it was, and there was much happiness for me; finally, a nickname I could get behind.
And "Su" it's been ever since.
(When I went to Google Images to get inspiration for this post's pic, I got a bunch of aircraft. Apparently, "Su" is also the abbreviation for Super-Fighters, or something along those lines. I also got a pic of Roger Federer, which is beyond odd, even while much appreciated.)