Maybe the whole idea is that the passengers don't know that we are preparing for an emergency landing, until either the Capt or the No.1 tells them.
I would guess that as soon as as the pax know, a number of them are going to hit panic mode (l am going to die). Now this will mean that some of the cabin crew will have to deal with the panicking pax and not getting their station or area ready.
This is only how l see it, but as l am not aircrew l may be well off the mark.