Nobody yet mentioned possible electronic glitch causing SFL to be zero...
Yes... and nobody mentioned EMP hit by North Koreans, alien abduction and static overload from too much balloon rubbing on their heads.
That electronic glitch must have knocked out the f/o, too. And the door mechanism.. or the five remaining crew members forgot the emergency code.
But to be fair, there may be all sorts of explanation... as there always is.
As we might never know for sure, we might stick best with good old Ockham:
"The simplest explanation is usually the correct one".
And by simplest I'd use the one with most known facts as of today.