Other than the reasons by the other comment, there is also the matter of sovereignty. In a single country divided into provinces, there is the King at the top and dukes serving each province, beginning the nobility hierarchy from there on, and the other comment applies. But consider an empire which has conquered different countries - some countries surrender without a war, and the existing king becomes a puppet sovereign with perhaps a duke as an ambassador, but in cases there is a war, the losing king and his family are ousted and most probably executed. Then the land may be given to a Duke to handle, making him effectively a king to that old nation - in such cases, at least for that country, he is royalty and therefore his children will be referred as prince/princess.
