Why hasn't Obama asked for Holder's and Napolitano's resignation?

Holder and Napalitano were involved in the gun running scheme which allowed 575 AK-47s get into the hands of Mexican Drug Cartels. One of these assault rifles was found on the scene where Border Agent Brian Terry was murdered. This makes them responsible for this agent's murder. Obama claims he did not know about the scheme (yeah, right) but IF he did not know about it then why hasn't he asked for their resignations?