The fictional setting of To Kill a Mockingbird, Maycomb is a sleepy little town with few entertainment possibilities for its citizens. A high-profile rape trial would have probably drawn a decent crowd to the courthouse under any circumstances, but this trial was different: An African-American man accused of raping a white woman was bound to be a hot topic. Not only did the townspeople attend, but the trial also drew people from the surrounding towns and rural areas. It was a natural reaction. (Public hangings were still lawful in some states during the 1930s, and they were known to draw huge crowds.) Like passersby who can't resist gawking at a traffic accident, the Tom Robinson trial was irresistible to most people living in the vicinity of Maycomb.
In my opinion, what this says about human nature is that we are sort of drawn to the gruesome things in life. It's like people slowing down to check out a nasty car crash on the freeway.
In Maycomb, the trial of Tom Robinson was going to be sensational for sure. It had everything that reality TV has and more. It had sex and it had violence, for example. In addition, it was likely to show someone getting put back in his place and people like to watch revenge taken on people they think are bad.
So it was like an interesting show with sex and violence and race but it was also like a car wreck because people could watch someone on trial for his life and likely to lose.