Retail investors have yanked money out of stock mutual funds for 17 straight weeks. And the still unexplained May 6 “flash crash” — when the Dow Jones industrials plunged more than 600 points in minutes before recovering — is increasingly being cited as a key reason the public has been selling. In a speech Tuesday, Mary Schapiro, chairman of the Securities and Exchange Commission, said the SEC was informed by retail brokers that the Main Street investors they cater to “have pulled back” from the stock market since the flash crash.