They're a major constituant of the Democratic Party and you don't get through the primaries without their endorsements, so their positions have much higher weight than your ballot.
When you see "Private Prisons Are The Worst!!!" check the studies to see if the groups are comparable; I know someone in the industry years ago and when they opened the first private prison in the state they were sent the worst offenders in order to try and break them, yet they ended up performing better than average for the state.
I understand that the prison guard lobby is terrible, but that's not the point. The question isn't "private prisons vs prison lobbying," it's "private prisons vs public prisons."
It's not the buildings that are a problem, it's the people running them. That's either the owners for private prisons or the prison guard union for public prisons. If you say "private prisons are bad for <these reasons>" but there's just as many problems (and mostly the same) with public prisons, then focusing your hate on private prisons isn't working on the real problem. The legislature sets the standards for both and could actually fix the situation, they have both the responsibility and the power to do so.
For all of its faults, I can't agree with you that the public sector is just as bad as the private sector.