[fiatmoney]

Suburbia is a defensive design. Big houses & lots, and lack of public transport, means that it tends to be more expensive and requires reliable access to a car. Low density means that blight doesn't spread as quickly, and geographically-selected organizations (in particular school districts) don't change as quickly, and they're also selecting membership from the same groups that can afford the real estate and transport. HOAs explicitly prioritize maintaining property values.

This all selects for higher-quality neighbors.

There are ways to achieve this in urban settings, but it tends to require more direct, onerous, and legally iffy regulation.

[InclinedPlane]

Wealthier neighbors. You meant to say wealthier.

[fiatmoney]

No, I didn't. The salient aspect is "nice" neighbors; people like poor teachers, or unemployed kids of existing residents, are just fine. Things like HOAs are particularly brutally enforced in poorer neighborhoods attempting to become or stay "nice" (eg, in black but predominantly middle class neighborhoods).