While John is not an unfamiliar name in English/British royal history, it had never been a name that amounted to much luck. King John had a disastrous reign where he lost some of his rights as king along with a lot of family land in France. Also two princes named John (one son of Edward VII and one son of George V) died young. Princess Diana had wanted to name her first son after her father John Spencer, but she was discouraged by royal tradition, so the family chose William instead.