Cardinal Sarah was given a job to do by Francis, himself. There were plenty of heterodox prelates he could have chosen but it was early days and he didn't want to be accused of turfing good Catholics out of of the Curiaall at once so he appointed Sarah but craftily sacked the rest of the Congregation leaving him Prefect in name only. Since then he has continued to humiliate this Cardinal, perhaps hoping he will resign.
He has destroyed the St John Paul Institute using the same underhand tactics. He has humiliated Cardinals Cañizares, Burke Müller Brandmüller, Caffarra, RIP and Meisner, RIP and many others in the Curia with his Christmas rants, Cardinal Sarah is in excellent company. If he is wise he will stick to his guns until Francis has no choice but to make another martyr.