Never forget the physicists: For every action, there's an equal and opposite reaction.

As the housing market blew up, millions of homeowners fell into foreclosure, and homeownership rates sank to the lowest level in a decade, you might have thought that demand for rental housing would increase, and rental prices would rise. That scenario makes a lot of sense. People need a place to live, after all. And if they're not owning, they're renting (the exception being homelessness, of course).

But this hasn't been the case. Rental prices have tanked right alongside home prices. According to apartment data analysis firm RealFacts, nationwide average rent was $933 in January, compared with $994 a year earlier. Apartment vacancies are near 30-year highs. As anyone who has recently shopped for a rental can attest, landlords treat prospective renters like kings -- free parking, free gym memberships, a few months' free rent, quiet neighbors (they all promise it), you name it. Just rent the darn thing.

This is a full-blown renters market. But why, when logic might suggest otherwise?

For one, renters are hurting like everyone else. Their assets have been demolished. Their savings have been drained. Their paychecks have been slashed. When renters simply can't afford to pay more, landlords don't have much pricing power.

But there are a few other, less obvious reasons.

Doubling up
When a bubble bursts and unemployment rises, you get swarms of renters who need to "double up" and find a roommate to save money. With two or more people living in one place, you can have increased demand for rental living while demand for rental units actually falls.

This phenomenon has been accentuated by the demographics of those hard hit by the recession. In general, rentals are occupied by a younger, lower-income, less-educated group. Unfortunately, this group has been particularly wrecked lately.

This might seem obvious, but the degree is pretty spectacular: As of September 2009, the average 12-month unemployment rate nationwide was 8.6%. Yet if you look just at those under age 24 without high school degrees, surely a group of mostly renters, the unemployment rate was a godawful 26.6%. In contrast, for those age 45 and over with college degrees, a group where homeownership is the norm, it was just 4.3%.  

So the more likely you are to rent, the more likely you are to be unemployed. And when you're a renter suddenly faced with a serious financial hardship, doubling up is often the best option. Another common form of doubling up involves new college grads who are unable to find work and move back in with their parents. Either way, the need for roommates has killed demand for rental units, which in turn kills rental prices.

Reverse condo conversions
During the housing bubble, it was cool to buy up apartment complexes, boot out the tenants, and resell the units as converted condos. This was especially prevalent in places like Miami and Arizona, where condo demand was insatiable.

But that was then. With the bubble thoroughly burst and scads of empty, unsellable condos everywhere you look, the conversion party has reversed. Owners of empty condo projects are now desperate to become landlords. As one 2008 article said of Arizona:

During the Valleywide condo craze, 30,616 apartment units were converted or sold to investors who planned to convert them to condos ... By the middle of last year, 18,000 units had reverted to apartment rental units and many investors had scrapped their conversion projects.

Obviously, this just floods the market with new rental supply, which can hammer prices.

Deed in lieu of foreclosure leasebacks
In a "deed in lieu of foreclosure" scenario, all financial rights to a property are handed back to the lender, without the homeowner technically going through foreclosure. All major lenders, Bank of America (NYSE: BAC), Wells Fargo (NYSE: WFC), Citigroup (NYSE: C), and JPMorgan Chase (NYSE: JPM), have such programs.

Last year, Fannie Mae (NYSE: FNM) began a deed-in-lieu-of-foreclosure program with a twist. After handing back the deed, the previous homeowner could lease back the property from its new owner, Fannie Mae. In effect, this turns once privately owned homes into government-backed rentals.

Admittedly, the impact this has on rental rates might be negligible. But it's another example of renters currently living in what should be, and typically would be, an owner-occupied home. All in all, that's bad for rental rates.

Comments? Rebuttals? Bring it in the comments section below.

Fool contributor Morgan Housel doesn't own shares in any of the companies mentioned in this article. The Fool has a disclosure policy.