(Not having the staff would seem to be a convenient excuse, but it could also be a very real thing. No word on who gets to pay (coughcustomerscough) for the overtime in these cases.)
Note that leaving in back doors for the Feds to monitor through apparently carries even more risks that it seems — some hackers monitor for just that, and try to exploit those same back doors.
Google, Facebook, and others could face fines over government wiretap refusals
A new bill being drafted by a secret government task force would impose fines on internet companies — such as Facebook, Google, gaming, and VoIP companies — if they don’t comply with court…

Reddit users think this story is a farce. IE the FBI wants people to think this.
I suppose it is possible they can't get in technically but surprising and good.