Why do you think we use the word "won't" as the contraction for "will not" instead of the more orthographically-compliant "willn't" or "win't"?
maybe it used to be willn't, but then in years and years of conversation, it got said shorter and started to merge conceptually with "wouldn't" or "don't". Also i can see why there would be a natural need for something that sounds more different from "will", its opposite