Because you already control the capital of the realm. In a sense, you already control the realm.
Think of it this way: the Duke of Edimburgh can secede Scotland. The Duke of Cardiff can secede Wales. But the Duke of London can't secede England, that's already the main part of the realm. He can seize power in all of England through a rebellion, and then Scotland and Wales can secede if they don't like it.
Dukes should be able to give a region to another duchy then.
