The suspects DID have backups of the phone on Apple's iCloud platform, and Apple already provided that to the FBI.
But that doesn't meet the FBI's actual needs. The FBI's ACTUAL needs are to have a case with a lot of public sympathy in which they can force a major tech company to very publicly comply with their order to add a backdoor to a phone (without calling it that) in order to influence the legal and legislative systems (and perhaps public opinion, if the FBI even cares about that).
Apple has provided the FBI with decrypted copies of the iCloud backups. But the phone only backed up "intermittently" so recent activity would not have been included in those backups. (Well, it COULD have been, except that the FBI told San Bernadino County to change the password, which messed that up.)
But that doesn't meet the FBI's actual needs. The FBI's ACTUAL needs are to have a case with a lot of public sympathy in which they can force a major tech company to very publicly comply with their order to add a backdoor to a phone (without calling it that) in order to influence the legal and legislative systems (and perhaps public opinion, if the FBI even cares about that).