It is usually economically advantageous for underwater mortgage holders to walk away. But, irrationally, they usually don't.

Most lenders will, in other words, take full advantage of the asymmetry of norms between lender and homeowner and will use the threat of damaging the borrower’s credit score to bring the homeowner into compliance.... On a fundamental level, the asymmetry of moral norms for borrowers and market norms for lenders gives lenders an unfair advantage in negotiations related to the enforcement of contractual rights and obligations.... This imbalance is exaggerated by the credit reporting system, which gives lenders the power to threaten borrowers’ human worth and social status by damaging their credit scores—scores that serve as much as grades for moral character as they do for creditworthiness. The result is a predictable imbalance in which individual homeowners have born a huge and disproportionate burden of the housing collapse.

—Underwater and Not Walking Away, Brent T. White. (emphasis added)