Many of whom don't have regular access to computers, or even their own individually assigned computer.
I believe they're required to log hours worked once a week online and would therefore have access to a computer to reply. A separate email was sent out either yesterday or today (don't remember which, I'd have to check my email) was also sent out to supervisors of civilian employees as a reminder of the deadline.
Some of whom are on leave, or furlough, or in the backcountry, or on special assignment, or etc.