Complex sentence: The Mozarts were free to leave Milan early in November, but they stayed another month because Leopold hoped that "Ascanio's" success would lead to an appointment for Wolfgang from a royal patron.
Split into simpler sentences: The Mozarts were free to leave Milan early in November but they stayed another month. The main reason that kept them was Leopold's hope that "Ascanio's" success would lead to an appointment for Wolfgang from a royal patron.