Why doesn't the young woman want to marry Scrooge anymore?

Belle breaks off their engagement because she does not plan on being second in Scrooge's life. She has come to see that his pursuit of money has become his main focus.... the most important thing in his world. Belle tells him that his love for her has been replaced by his love for gold. Thus, she ends their relationship.

He is beginning to love money and value it’s importance more than love