Apple had been working with the FBI to use the only available recovery method which relies on iCloud backups but when the FBI or some other law enforcement expert switched iCloud accounts for the phone that recovery method went by the boards and cannot be recreated. The iPhone 5 and 6 along with iOS 9 are by design far more secure than previous ones.
It was reported in today's San Francisco
Chronicle that the iCloud password reset was done by the FBI to prevent remote wiping of the phone data. The article said the government (at that time) did not know if someone else knew the shooter's password and would try to erase the iPhone's contents.
Remotely erasing an iDevice is something any owner (or person with the correct password) can easily do over the Internet.
