Why do old city neighborhoods get rundown?

Answer 1

The obvious answer is "Because without maintenance and occasional renovation everything old gets rundown." Of course that avoids the issue of why old neighbourhoods tend to enter a phase where everyone stops doing that maintenance and renovation. To a large extent, that's because of real estate depreciation. Even with maintenance, property values tend to steadily go down over time. As the price to live there gets cheaper, more and more people living marginal existences move in and with them come the people who prey upon those people. Those people tend to be less likely to maintain their property so they neglect it and of course that drives real estate values down even further and the more entropy they see around them the more people are inclined to just give up and let it go. This trend tends to continue until something happens like artists making an area hip, or a real estate guy taking advantage of the low property values to do some king of major development that will level most of the neighbourhood and push the marginalised inhabitants into some other neighbourhood on the downward slide, continuing the cycle.