Tom's trial was particularly difficult for Atticus to win, as the racial codes of the 1940s made it very likely, from the beginning, for Tom to be blamed, even when there is a lack of evidence. These are some of the reasons why this is the case:
The social context of the South is one of prevailing racism. Although not all residents are racist, most people do internally believe that white people are generally better than black people. This means Tom's trial is difficult from the start.
There are sexual undertones in the book, as Tom is accused of ra.pe. This reflects the idea of African-American men as being overly sexual and interested in white women. This is reinforced by the fact that Mayella is not seen as a woman who might have desires of her own.
Finally, the fact that African-Americans in general were not wealthy means that Tom is not only black, but he is also a farm labourer, without much wealth and status. This places him in an even more vulnerable position.
Answer: Racial codes make it impossible for Tom to win trial because white people were extremely racist back then and even the judges were bias in favor of white people. In the book This is demonstrated by Tom losing the trial with no factual evidence against him. Lastly even a black person showing their feeling of pity or remorse for a white person angered white people back then because they felt they were being looked down upon.
