[ptero]

A few random guesses (I am not in any way affiliated with FB); just my 2c:

Sharing status of an active event may complicate recovery, especially if they suspect adversarial actions: such public real-time reports can explain to the red team what the blue team is doing and, especially important, what the blue team is unable to do at the moment.

Potentially exposing the dirty laundry. While a postmortem should be done within the company (and as much as possible is published publicly) after the event, such early blurbs may expose many non-public things, usually unrelated to the issue.