I have to disagree with your characterization of people in suburbs. The only anecdote of trusting behavior given in the article was of a rural farmer in an isolated area - and it was given in contrast to people from urban New York or New Jersey coming up and being amazed about it.
Here's another hypothesis - tribal or rural cultures will be more trusting by nature. Human beings are naturally paranoid but NOT of each other. If you are competing with other species and the environment for survival, it doesn't make sense to view your own species as a threat. Indeed, it benefits your family or tribe if you all cooperate. However, in densely populated urban environments, your competition is all human beings. If you are metaphorically eaten or taken advantage of, it will be by another human. It's much less likely in a city that you will die because of exposure, or weather or a wild animal. Thus, because the threat is from people, you must be more wary of people.
I'm not super confident about this of course - it's probably both simplistic and wrong on a few levels, but I think whatever the answer is, it's not just that urban people are trusting and rural people are paranoid.