Sometimes I lie awake at night and wonder
Where my life will lead me
Where the years have gone
Why you say won’t and not willn’t.
I just destroyed a mellow song by Jackson Browne, but it was not for naught.
Few other words (other than irregular pasts, more about that later) are ballsy enough to just switch from “will” to “wo” and just hope to get away with it. I mean, I know this is fair game in languages like Arabic, where they only write the consonants and a few vowels and every word is like a game of Wheel of Fortune where you buy the remaining vowels. Or you don’t, but then you’d basically be speaking Tunisian.

Anyway, English has a set of clear rules (for a change). “Is not” becomes “isn’t”, “he is” becomes “he’s”, “should not” becomes “shouldn’t” and so on. Why not “willn’t” then?
Alas, if you look into the historical background “won’t” doesn’t turn out to be the rebel I have made him out to be. “Won’t” is not a contraction of “will not”. It is a contraction of “woll not” or “wol not” or “wonnot” (source).
Language speakers back in the day could not really make up their mind (as I struggled with deciding on what to have for breakfast this morning, I somehow feel I can relate) on which word to use to express what we now know as “will” (source). They used “wulle”, “wole”, “wool”, “welle”, “wel”, “wile”, “wyll”, and even “ull”, and “ool” (am I the only one that is sad “ool” is not around anymore?).
So basically, these forms grew apart after a while, and in the language divorce “will” got the positive sentences and “woll not” got the negative sentences (“ool”, sadly, was left empty-handed).
Try saying “willn’t” five times and you’ll agree it was for the best.