We do not say would you because that would suggest it will happen on conditions. "Would you marry me - if what?" We say will you because we want it to happen in a yes or now situation without conditions; will this or will this not happen,,.

"Would" is conditional upon something. Would you marry me if I were rich? Would you marry me if I were better looking? etc....

When we say "would you" it is as if we wewe asking an opinion like," would you like an apple or an orange? "or " would you like a lemon or an orange" or "Would you prefer to eat outside or in?" or would you marry me or another guy? When we say "Will you "it is more positive, it requires a single answer, Yes or No. Hope this helps you