It's really a combination of things. Most recently it is the weather. I've been back about four months now and the numbers we're definitely higher on the cold days when the sun sets at 4:45pm. It's just hard being inside playing a video game when it's nice out day and night.
The game is definitely repetitive, but making friends and flying different aircraft help. I'm still blown away that in my ten year break there's only a couple new maps. I fully expected to come back and find 20+ maps in rotation, in seriously shocked how few options we have.
The game definitely isn't beginner friendly. Super high learning curve, lots of key mapping, and a highly skilled player group in the MA definitely makes it hard for noobs to get hooked. I really think HTC should create some tutorial missions before you fly online.
If you look at the numbers during a special event, there's clearly a decent amount of people that keep accounts but don't participate in the MA. I bet tonight's FSO is about 180 pilots and the MA will have 150 or so.
Anyways don't let the numbers keep you from returning, it's worth it and action can be found.