I suppose it's governed by the same sorts of outdated rules that our Kings and Queens are appointed by. There'll be all sorts of rules about 'marrying out' and first born boys and whatnot, I would have thought.

The prophesy - there must be more to it than we realise yet, as the reason Voldemort chose Harry over Neville is pretty unclear...at that age, who could tell how powerful a wizard either of them would become? There's something missing here.