The Greater Good
This is because of a decree that King George V issued in 1917. To be specific, he made this announcement, “The grandchildren of the sons of any such Sovereign in the direct male line (save only the eldest living son of the eldest son of the Prince of Wales) shall have and enjoy in all occasions the style and title enjoyed by the children of Dukes of these Our Realms.”
Her Duty
Unless a male royal is directly in line to the throne, he is not a prince in essence. However, the “male” aspect of this ruling has been changed. Before William and Kate had their first child, the British government began following “absolute primogeniture.”