It wasn't wiped, it was disabled. They actually did get the iCloud backup data, but from about a month and a half before the phone was seized. Instead of waiting for another iCloud backup to occur, they reset the password, which disables these backups. The FBI then lied about this, saying that resetting the password was done against their will by the county. The county disputed this claim, and the FBI eventually admitted that they had actually instructed the county to do so, leaving open the question of why they did this.
source: just search "San Bernardino iCloud" and you'll get a whole bunch of articles on the subject
