He argues that one of the biggest factors of the prison boom is with the DA having to be 'tough on crime' to further future political ambitions. I don't think anyone can argue with this, as historically being 'tough on crime' has been a vote getter.
A few other factors I think are worthy of considering:
1) The financial incentives of the for profit prison system (which includes for profit prisons, prison guard unions, bail bond industry) which advocates for harsher sentences.
2) The political incentives of having a larger prison population. Many congressmen fight to have a prison in their district for two main reasons. First, it brings jobs to their community. Second, and more importantly, it increases the count of people who they 'represent' even though the felons are not allowed to vote!
3) The criminalization of mental illness- We used to heavily fund mental hospitals. This has been chipped away dramatically starting in the 1980s.
4) Average lifespan of prisoners increasing- I would assume that the average prisoner's lifespan is increasing, hence a bigger prison population. No real evidence to back this up.
5) Drug criminalization leads to more criminals- The professor argues that the population who are in prison for drugs alone is only a small percentage, and that the rest are due violent crimes and property crimes. This is misleading as drug criminalization naturally leads to more criminal activities in other fields. By pushing drugs underground, a group of people will be inevitably become comfortable with the underground. Secondly, drug criminalization leads to higher drug prices, which leads to more property crimes to fuel addiction. Lastly, drug criminalization means people are less likely to get help for their drug problems, increasing the chances they will become socially maladaptive and more likely to develop violent behaviors.
6) The gutting of public defenders- Tied to what the professor argued but surprisingly not really mentioned. Public defenders are extremely underfunded, hence a high likelihood of losing your court case. This in turn leads to a higher prison population.
7) Discrimination against prisoners- As our economy gets more and more competitive, and as background information becomes easier to obtain, it is getting harder and harder for prisoners to get a job. Tied to point #5, a person who would be arrested for drugs could have his future economic career ruined. He has a higher likelihood of reverting back to crime, hence a higher prison population.
8) The removal and policing of public space- One thing I noticed is that public spaces (i.e. parks, stoops, etc) have been routinely removed to make space for private retail. It's way easier to call the cops when you are angering the property owner than if you are on public property. And in those public spaces, you are subject to way more laws (i.e. public intoxication). So it should be no surprise that we have a higher prison population if we're systematically policing more and more areas.
Many of these problems stem back to Reagan in the 1980s with America took a decidedly right-wing turn. Glad to see society is finally changing what it thinks about the over-criminalization of society.