It's not, and they did. What I don't understand why so-called "reporters" are regurgitating the blatant trash from "unnamed FBI sources" as if it's fit to print. I would hope at least to see front page NYT and WP editorials blasting the FBI for their obvious failed attempt here to capitalize on a mass murder for their own legal/policy purposes.
A few weeks ago they couldn't figure out how to unlock the phone, when it's blatantly obvious the security weaknesses of the 5c, and today they suddenly have seen the light but it just needs more testing? Department of Homeland Security has been funding research into NAND-blockers for at least 6 years now.
In a fair and adversarial system, the court would somehow sanction the FBI for their prior sworn testimony that they couldn't do this themselves. But of course when Federal agents lie under oath, it's not a crime, just a misunderstanding.