I believe it has to do with the frequency of letter use (if that makes any sense!) The most often used letters are placed within "easy reach". I could be wrong though. Why don't you do a search?
Has more to do with the mechanics of a traditional typewriter, and keeping the typebars from jamming with common keystroke combinations. Learn something new everyday!
Very interesting stuff. I found the history of the "shift" key to be pretty amusing. I mean, I guess when you hold it down it still shifts the functions, but doesnt physically shift them as it used to. Very good stuff