I don't think poor people in the US would assume they win. And that is one major factor: when there is little to nothing to gain by staying Wwithin the system, but a lot to lose, why would you stay?

Another issue is that showing up isn't free. At the very least, it costs time. Remark that defendants typically aren't compensated, even if acquitted. If you're poor, you might not be able to afford to attend.