Why?

Ban stopped a moment upon hearing her suggestion. It seems that she was really adamant on him meeting her father. Ban had already started to suspect that she must be up to something. 

"Maybe next time, I still have something to do later." Ban decided to reject her flatly. 

"And besides, I did not save you and your Monster for the rewards, I just felt like it was the right thing to do at that point…" he explained the truth. 

He was really not thinking about the rewards earlier, saving her was just something he did because he was afraid that her father might pursue him to the depths of hell if she did not return in one piece. 

Belle was dejected upon hearing his refusal. However, her mood immediately brightened the moment Leon started speaking the second half of his speech. 

"I-I understand… but I would still ask my father about it…" she said with a playful smile.