There aren't a lot of people on during the event as in the past. This is evidenced by the users online count throughout the day. In past years I've seen it easily spike to over 1k while an event is active, but lately I haven't seen it break 700.
In my opinion subeta should send a newsletter to let people know survival has started and is fairly stable now, due to it's later-than-usual startdate and error that turned people away from the forums at first.
In the past, survival has started around the 14, 15, 16 - but this year it started on the 19th. We had no news/public word about the holiday until the 18th. Casuals could have turned up around the 14-16th, saw no signs of survival, maybe assumed it was canceled, and left again. A newsletter could be necessary to bring people back in.