[eschaton]

In depressed Rust Belt towns, basically everyone who isn't destitute or living in an area for a short period of time (like a student) has the opportunity to buy a home somewhere. Hence you would expect that median renter income in those cities would be much lower than the total median income. In contrast, in an NYC or San Francisco, you wouldn't expect anywhere near as dramatic of a gap between median renter income and median household income.