Colin Powell used a private email server and so did Condoleezza Rice when they were Secretary of State.  So if Clinton asked Powell what HE did - that would be what he told her.  

Is it really surprising that Clinton would want to protect her Privacy?

When she was First Lady to President Bill Clinton the House Republicans tried to punish her and also the President for opposing them through the use of a Special Persecutor investigating their Whitewater Real Estate dealings.  When NOTHING improper could be found - the Muck-Suckers found a bitter Linda Tripp to Rat Out a poor Intern that had told her she had 'scored' a BJ off the President and had kept a Stained Blue Dress as a Souvenir.