Why are emergency-instructions by cabin crew like "heads down!" not given via PA? I just saw a YouTube video of a landing airliner with the gear not fully extended and the cabin crew gave the instructions "Stay down! Heads down!" repeatedly to passengers but without any technical help like PA. Why is that the case? Why don't they use the PA system for commands like that?
Not knowing that specific video, impossible to tell the scenario.

But in general, you want to not have to rely on technology in an emergency situation. What if you were trained to use the PA system but there is no electricity?
So you train to shout and use non-verbal communications to get your point across in an emergency.