Politicians are elected on so many different issues that it's really hard to get enough people to care enough about one specific thing in order to have elections actually mean something.
Fictional numbers:
90% of the population cares about Issue X, but on a scale of 1-10 in how important it is to them, it's a 3.
Meanwhile, 45% of the population cares about Issue Y, but to them it's a 10.
Issue Y is what people will choose their politicians for. It's natural selection. I still have an appendix because, while it's useless at best and dangerous at worst, it's never killed enough people to make it a threat to the survival of the human race. Likewise, people are going to vote on war, abortion, religion, economics, civil rights, etc. They care about the prison population... they just care about other things more.
Not to mention that fixing the prison overpopulation issue isn't just passing or repealing one law. It's a multitude of laws, it filters from Congress through the judicial system to the police officers. See how long it's taken to get marijuana decriminalized in just a handful of states? Now that's just one injust thing that people get sent to prison for in the US. Trying to fix the rest of them will take even longer.
You don't get elected on "reducing the prison population".
You just described the problem exactly. Instead of getting elected based on principled issues such as "America's prison population is out of control. We should reform the justice system to easier for citizens to understand", the only way to get elected is target some special interest group. You stand a better chance of getting elected on some relatively unknown issue that is relevant in a handful of people's lives than something based on principles.
The majority of Americans do not care at all about politics. Ignorance is a rational choice in the face of the enormity of the American Federal government alone. They don't care that if you started reading the Federal Register for last year today, you would die before you finished. They don't care that if you tried to be a good citizen and understand just the tax codes that apply to you, you'd never figure it out because it changes faster than you can learn it.
Fictional numbers:
90% of the population cares about Issue X, but on a scale of 1-10 in how important it is to them, it's a 3.
Meanwhile, 45% of the population cares about Issue Y, but to them it's a 10.
Issue Y is what people will choose their politicians for. It's natural selection. I still have an appendix because, while it's useless at best and dangerous at worst, it's never killed enough people to make it a threat to the survival of the human race. Likewise, people are going to vote on war, abortion, religion, economics, civil rights, etc. They care about the prison population... they just care about other things more.
Not to mention that fixing the prison overpopulation issue isn't just passing or repealing one law. It's a multitude of laws, it filters from Congress through the judicial system to the police officers. See how long it's taken to get marijuana decriminalized in just a handful of states? Now that's just one injust thing that people get sent to prison for in the US. Trying to fix the rest of them will take even longer.
You don't get elected on "reducing the prison population".