[chmod775]

Because you now have more infrastructure than people to pay for and maintain it. As population density decreases, the people that remain are using oversized infrastructure. The relationship between required maintenance and amount of usage is far from linear. For a lot of infrastructure being underused means additional maintenance procedures become necessary.

A way to combat this is to move people and condense the population again, abandoning areas in the process and pretty much writing them off. Not optimal, but it'll help.

[Pay08]

I've assumed that moving people in order to consolidate infrastructure use was already modus operandi in countries that need to worry about these things in the short term, but it makes sense that it doesn't work that way.