It was because adding one to each of the letters in UNIX yields gibberish, but adding one to each of the letters in VMS gets WNT.
Era-appropriate joking aside: There's no actual evidence that Cutler held the views on Unix, or even on DEC's Eunice, that have been ascribed to xem from anecdotes by Armando Stettner and edits to Wikipedia and writing by G. Pascal Zachary. I and others went into more detail on this years ago: https://news.ycombinator.com/item?id=22814012
Era-appropriate joking aside: There's no actual evidence that Cutler held the views on Unix, or even on DEC's Eunice, that have been ascribed to xem from anecdotes by Armando Stettner and edits to Wikipedia and writing by G. Pascal Zachary. I and others went into more detail on this years ago: https://news.ycombinator.com/item?id=22814012