They all anticipated how the man would answer Li Zi's question. It's understandable that the first question any abandoned child would ask is one which begins with "why", why they were abandoned?
The man was having a hard time answering the girl's question. This was not part of their plan at all. They were only asked to stand quietly and let Yuexi An and the woman talk. He is father who would never abandon his children even if it means getting himself killed. But right now, he is acting as this girl's father, a father who abandoned his child.
So the question now is, how, how will he answer her?
"I'm sorry..." The man answered as such for he felt guilty not because he abandoned her as her father. Instead, he felt sorry that he needed to lie in front of his own daughter. The girl is innocent and he can't bring himself to lie to her. He would rather apologize than make stupid and silly excuses to justify himself.
