Tom Robinson could not defend himself because of the differences between the accused and the "victim." To put it straightforward, Tom could not have defended himself because of the fact that he was a black man and Mayella was a white woman. Society wanted to keep as many rights away from the black community as they could at the time, and that included fair trials. Even though Tom and Atticus had a fair and firm argument against her accusations, the jury was biased. Every time Tom tried to say that the "high and mighty" Mayella was lying, he would dig himself into an even bigger hole.
I hope that covers it. If you have any other questions regarding the subject, please don't hesitate to ask!
- Akobel