I don’t pretend to. I know the basics. But something 3 year old Imogen says has made me think.
For will not, she says “willn’t”. I thought she was wrong, but that it made perfect sense. More sense than “won’t”. What it says to me is that she has a good understanding of how our language works.
So I looked up willn’t. Apparently Charlotte Bronte used it in “Shirley” as it was local Yorkshire dialect. Perhaps it still is! Imogen is one quarter Yorkshire.
At one time in some parts of the country people said wol instead of will. But as the language developed we seem to have hung on to the wo bit for the negative and will for the positive.
Conclusion – it seems that willn’t is the archaic form of won’t and it may just be as simple as won’t being easier to say than willn’t.
And upon further investigation, I find there is even a Facebook page dedicated to willn’t. It is called "Willn't, the grammatical contraction of the future".
I think I prefer won’t, but this is the joy of having a living language. It could change back to willn’t one day. And why not? My spell checker doesn’t like it, but Imogen does.