The worlds too big, everyone is writing their own story, there's no hooks or events to draw people together. 
The're not creating it on the their own so the GM's need to guide it.

That can be hard because everyone is districts apart.
There should be a pre-requisite in character creation how you know at least two other characters, forcing character collaboration.

I don't think flooding the RP will get the results but at the same time you need dedicated players.


If I was GM'ing this I would look at throwing some NPC's at players to rebuild interest, then try tie the individual stories together.
Just thoughts :/