5 OED-cannot definition: the ordinary modern way of writing can not Both cannot and can not are acceptable spellings. There’s no difference in meaning between cannot and can not. but the cannot is much more usual. Some references from ELU - can't vs cannot Why is “cannot” spelled as one word?.. (a few exceptions)
Cannot is the only negative form that contains not rather than -n't. Theoretically, since it is a single word, you can say why cannot you... without a problem. My theory is that modern English speakers don't want to put cannot before the subject because it contains not and sounds like can not.