So why not now?
According to reports, the phone had not been backed up for 6 weeks. Apple provided that data to the FBI weeks ago.
The phone should automatically back up to iCloud if it is plugged in and connected to a known WiFi network. Apple recommended to the FBI that they place the phone in the terrorist's work office where it could connect to the office WiFi, and plug it in so that the iCloud backup would be triggered. However, when the FBI had the Apple ID password changed, the password on the account and the one saved on the phone no longer matched - so iCloud would no longer back up until the password was changed on the phone (which they cannot do because the phone is locked)!
Thanks
Unless the phone is set to NOT back up automatically... And unless you subscribe to more storage, most iPhones that DO auto-back up only keep contacts and calendars backed up...