Q: 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"?
A: 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