Home Loan Deficiency Judgments

Most discussions in the media of homeowners whose mortgage balances exceed the value of their homes proceed on the premise that an option available to those homeowners is to walk away from their mortgages. However, it appears to be the law in many states that a lender may obtain a deficiency judgment for the amount by which the loan value exceeds the amount realized upon foreclosure of the mortgage. Why would lenders not seek to obtain and enforce such deficiency judgments where there is at least some prospect of even partial recovery on them? It hasn't been traditional that borrowers can escape liability on unsecured debts. So why on these? And such judgments generally last a long time, so even the prospect that the borrower might some day come into some money should be enough to encourage lenders to pursue them.