In April of 2009 the U.S. Congress, under pressure from the banking industry, persuaded the Financial Accounting Standards Board (FASB) to delay implementation of mark-to-market accounting for mortgage related assets.* This delay provides banks with the opportunity to continue carry mortgage assets at origination value rather than at their current market value. The conditions that would force the revaluation of mortgage assets would be completing a foreclosure or taking the property in as a REO. It seems banks and mortgage investors are delaying foreclosure proceeding and resisting taking back properties in order to avoid having to reprice their mortgage assets (which would show the true state of their balance sheets and force banks to add to reserves).

Many mortgage borrowers are far in arrears on their mortgage payments, well beyond 90 days. The banks do not want to take possession of the properties and thereby have to deal with the costs (property taxes, security, HOA fees, and municipal assessments) of owning these homes.
* http://online.wsj.com/article/SB124396078596677535.html