With rents rising and incomes shrinking across the country, the Chicago-area has fallen behind other major metropolitan areas in creating new housing for its poorest families during the past decade.

But why?

It’s a question that jumped out at us while checking out some interesting new research by a Washington D.C.-based think-tank, the Urban Institute. We were using this interactive map to see how Cook County stacks up with other big cities when it comes to matching “extremely low-income” households with rentals they can actually afford.

In Cook County, which is anchored by Chicago, the number of households that fit the extreme poverty status (that means a household of four is getting by on $22,750 or less each year, according to the U.S. Department of Housing and Urban Development) hasn’t exactly grown by leaps and bounds since 2000. Yet with demand relatively steady, the county hasn’t closed the gap at all. Instead, it’s widened.

In 2000 there were 41 apartments available for every 100 households in need, according to the UI analysis of Census Bureau and other federal data. By 2012, the ratio had fallen to 26 out of 100.

Erika Poethig, the director of urban policy initiatives at the Urban Institute who co-authored the report, says there’s no one reason for Chicago’s slide. As a former Chicagoan, she did have some great local insight on the topic. As she walked us through some of the factors, it was like taking a trip down memory lane in The Reporter archives.

The Chicago Housing Authority’s Plan for Transformation accounts for the loss of thousands of units, Poethig says, though she points out that many of them were substandard and empty back  in 2000 when demolition began. The CHA has been slow to replace many of those public housing units — particularly ones large enough for families. The loss of units has been compounded by gentrification and the fact that private landlords are opting out of federal subsidies (or are running their subsidized buildings into the ground). And the foreclosure crisis was a death knell, creating thousands of vacancies and sending unwitting renters packing. Add it all up and what you’re left with is the growing gap revealed by the Urban Institute.

Unlike the Chicago-area, some places are actually closing the gap. Los Angeles County, for example, added more than 34,000 units for the county’s poorest during just more than a decade. New York City’s five counties combined to add another 30,000.

If there’s one take away from the Urban Institute’s report it’s if public officials decide that improving housing prospects for their poorest residents is a priority, housing can be built. Only time will tell if Chicago will try to catch up.

is a staff reporter at The Chicago Reporter.