It's a political move. The FBI wants this capability for all iPhones. But this particular case is very difficult politically to argue with to the general public

It's a political move. The FBI wants this capability for all iPhones. But this particular case is very difficult politically to argue with to the general public. The FBI realized they had a strong political force available to them, and they seized it. They are attempting to either force Apple's hand, make them give into public pressure, or--best of all for the FBI--to convince lawmakers to pass a law requiring companies to provide backdoors. The FBI will gladly keep sealed its requests that Apple unlock phones in more ambiguous cases, because it would be easy for Apple to publicly refute. But this case is just too good for the people at the FBI who want keys to everyone's data.