Years of Tradition
Such rules have been part of their tradition for a long time. Royals also don’t tell the baby’s identity to the public. After all, the royal family has been around longer than sonograms. Even Prince William and Kate Middleton are said that they did not want to know their children’s gender.
The Queen First
The Queen should get to know first if they got pregnant. She outranks everybody as the monarch, anyway. When Prince George was conceived, Prince William called the Queen on an encrypted phone. But soon, the public heard the story and after the duty was fulfilled, Kate’s parents will be informed.