Treasury Secretary Timothy F. Geithner and other top regulators were given sweeping powers after the 2008 financial crisis that would allow them to force new rules on money market funds, a popular type of mutual fund that has taken some of the blame for the crisis. On Wednesday evening, the head of the S.E.C., Mary L. Schapiro, unexpectedly announced that she was calling off her agency’s long-running effort to change rules for money funds.