Halliday & Matthiessen (2014: 174-5):
In the WH- interrogative, the negative is more variable. It is common enough with why, especially in contexts of disapproval; e.g. Why didn’t you tell me before? With the other WH- items the negative is more restricted. It does occur straightforwardly as a question, e.g. Which ones don’t contain yeast?; and especially perhaps in questions of the echo type: They didn’t have any bananas. – What didn’t they have? Otherwise it tends to function as the equivalent of a generalised positive:
I’d love to live in a house like that! – Who wouldn’t? (= ‘Everybody would.’)