Halliday & Matthiessen (2014: 101-2):
Interrogative clauses, therefore, embody the thematic principle in their structural make-up. It is characteristic of an interrogative clause in English that one particular element comes first; and the reason for this is that that element, owing to the very nature of a question, has the status of a Theme. The speaker is not making an instantial choice to put this element first; its occurrence in first position is the regular pattern by which the interrogative is expressed. It has become part of the system of the language, and the explanation for this lies in the thematic significance that is attached to first position in the English clause. Interrogatives express questions; the natural theme of a question is ‘I want to be told something’; the answer required is either a piece of information about an element of the clause or an indication of polarity. So the realisation of interrogative mood involves selecting an element that indicates the kind of answer required, and putting it at the beginning of the clause.