Although the Civil Rights Act of 1875 improved conditions for African Americans, in the South Democratic whites gained control in the South and proceeded to pass legislation that disenfranchised blacks as well as poor whites. There were lynchings and vigilante violence in the black communities and both the poor whites and blacks were exploited as sharecroppers. These conditions were still in existence in the 1930s in the South; however, in the North the Harlem Renaissance was yet flourishing since its incipience in 1919. Many blacks from the South as part of the Great Migration came to Northern cities for work in factories and other places.
In the South the Jim Crow Laws kept African-Americans segregated. There were no black high schools, so black children did not advance past eight grade; most blacks were sharecroppers or worked lowly jobs. With the onset of the Great Depression, blacks found themselves in even worse economic situations as the growing number of poor whites took the menial jobs such as that of picking cotton. In addition, the vigilante practice of lynching and terrorizing was still in practice in the 1930s.
It is because of these existing conditions that Tom Robinson is fearful of coming into the house of Mayella Ewell. When her father discovers him on their property, Tom runs in terror. As he is on the witness stand, Tom verbally expresses this dread of lynch mobs when he tells Atticus Finch,
"I was scared, suh....Mr. Finch, if you was a n*** like me, you'd be scared, too."
After the trial and his conviction, Tom is too fearful of the mob to wait for an appeal; instead, when he sees a window of opportunity, he runs. With his being in such a setting, Tom's actions are reasonable.