Yet another terrible thing about prisons - they target policing in urban areas where the population is high and tends to vote Democrat. Once arrested, tried, and convicted, that person is sent to a prison in the middle of nowhere - and now counts toward the population of that area, but cannot vote themselves.
Population is used to determine how congressional districts get allocated, and they haven't increased the number of congressional seats in decades. So as the prison population rises, they are gaining seats at the expense of urban areas losing the same number.
It's literally an updated version of the 3/5 compromise.