Contractions in questions

[enunciativo]

I've always considered it odd that we say "Why didn't you enjoy the book?" for the contraction-free "Why did you not enjoy the book?"  Obviously, we can't split the contraction by inserting the subject, and no other contraction is possible;  however, I don't see why the desire to include a contraction overrides the normal syntax.     

[freknu]

In modern standard English, "did you not" is indeed the correct way, but that is a modern phenomenon. E.g. German and Norse do not need to use that order but can also use "did not you" just as well. English used to do so as well — and dialects may still have retained that feature — but in the modern language it is no longer used.

[Daniel]

This has actually been discussed quite a bit within research on clitics (morphemes that are between words and affixes, showing properties of both). Here's at least one paper that discusses -n't in English:
http://web.stanford.edu/~zwicky/ZPCliticsInfl.pdf

[enunciativo]

Thanks, Freknu and Daniel.