Explain why the mob outside the jail does not finish what it came to do in To Kill a Mockingbird?
I'm wondering if your question has to deal with Prisoner of Psi or, possibly, with the novel To Kill a Mockingbird. I will provide you with an answer regarding TKAM.
The lynch mob goes away empty-handed because of the unexpected appearance of Jem, Scout and Dill. The mob probably knew Atticus would be standing guard (or maybe not). They were willing to forcibly remove him, if necessary, but the group from Old Sarum weren't prepared to deal with the three innocent children. After one of them tried to forcibly remove Jem, Scout kicked the man in the groin.
I intended to kick his shin, but aimed too high.
After that, Scout's innocent dialogue with Walter Cunningham Sr. turned the once-deadly mob into a group of "impassive" men who were shamed into the reality of their deed by a young girl. The mob may have been ready to kill a black man, but they weren't ready to do it in front of a group of children who were classmates of some of their own kids.