The reason is that they probably deleted everything on it considering the ineptitude they have shown from the start.
First FBI Agent: "OOPS!"
Second FBI Agent: "Uh, let's not tell the Agent in Charge that we tried eleven passcodes. What he doesn't know won't hurt US!"
Third FBI Agent: "Hey! I've got an idea! Let's get the San Bernardino IT guys to change the AppleID and see if that will let us get in!"
First two FBI Agents in unison: "GREAT IDEA!"
Third FBI Agent: "Let's go."
SB County IT Guy: "I don't think that's good idea."
Third FBI Agent: "We heard on the Internet this would do it. Go ahead. We're the FBI, you have to do it."
SB County IT Guy: "Why don't we call Apple and ask them what to do?"
First FBI Agent: "Look at this FBI wallet, that's all the authority you need to do what we tell you! DO IT!"
SB County IT Guy: "Well, OK, but I still think you should ask Apple. . . OK, done. " Third FBI Agent: "Try it. . . "
All three FBI Agents: "OOPS!"