Why do we say won't instead of willn't?

Question

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?

Answer 1

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.