The duchy of Brittany operated more or less independently from the French crown and was only incorporated into the monarchy when Anne of Brittany married Louis XII of France and her daughter, Claude, married Francis I. Additionally, the duchy of Burgundy was a constant thorn in the monarchy's side and was only absorbed into the Hapsburg empire when Mary of Burgundy married Maximillian I, Holy Roman Emperor and their grandson, Charles, inherited both.

Meanwhile, in England during the same-ish time, the dukes of Norfolk, Buckingham, Suffolk, Somerset, etc acted as full subjects of the king and several were flat out executed by Tudor monarchs.

So why was there such a difference in agency between French and English dukes?