If won't is the contraction of will not, where did the "wo" come from in won't? Why is this convention over willn't?
Won't is a contraction of "woll not", woll being an old synonym (or alternative form) for will.
Why hasn't "willn't" (or "amn't" for that matter) become accepted? I am tempted to attribute it to euphony, but that seems rather pat.