Because the realm is tainted.
What is tainted that can't be removed by dealing with the nobles? A realm is only the sum of its nobles. The fact that recently their history is unacceptable? Such will fade in time, especially if you deal with the nobles within the realm.
They need to capitulate. Otherwise it makes no sense to just hand over regions. The capitulation should be more than symbolic.
Publicly surrendering, with all the acknowledgements of 'we done wrong', and having the government members step down to put a new government into power (whether of their own choice, your choice, or something in-between), isn't non-symbolic capitulation?
A new realm would start fresh and wouldn't have the taint from Fulco? Was that his name? Old ruler in Darfix who has made poor choices and committed betrayal more times than I can remember.
And what will the history of an old ruler matter with new government and direction?
