Different cities will all have different reasons for their barrens, but once you mix together poverty, no way out (due to lack of SIN), and high levels of crime, gangs, and addictions, the effect will be pretty similar, and pretty similarly stubborn to clean-up. So in most places, by 2075, I think you can just say that the barrens just are, and most people don’t remember a time when they weren’t there, or how they got started.
But if you need reasons, there are plenty in the NAN too.
- In the various NAN nations there are probably plenty of SINless, as not everyone left who should have at the creation of the NAN states, and their descendents are probably just as screwed as the SINless are anywhere else. I don’t have the NAN books from previous editions, so I don’t know, but I assume that in at least some of the NAN states some of the non-natives would end up with a sort of second class citizenship, which could also limit their opportunities and lead some to ending up in barrens.
- There was probably also economic immigration into the cities as most of the NAN would have had pretty collapsed economies at their creation (massive economic, demographic, and legal disruption, lack of credit to keep businesses running, etc.), and people who are broke and hungry often move into cities where they hope there will be more opportunities.
- And then, sadly, people who grew up on some reserves would have inherited multiple generations of poverty, lack of education, and high levels of addictions, and many of them may have moved into cities (especially capitals) hoping for better -- only to find that this inheritance made it hard to get ahead anywhere.
- Goblinization was a problem that no nation was ready for, and almost anywhere it is likely that some of the new orcs and trolls would have fled or been pushed out and formed their own gangs and/or ghettos, which in turn will tend to keep the entire area around them economically depressed.
- A lot of the new NAN governments were not particularly savvy in business or legal matters, and were economically desperate early on, so may have signed deals with corporations which resulted in fairly toxic areas being left behind – at least poisoned enough to keep away people who have options, which in turn will tend to cause the desperate to congregate there, and between the poison and the poor it will tend to pull down adjacent areas.