I don't think it's right to kill off characters or take them over. I'm going to just say that the characters that are no longer active just sort of "walked away." 

I need everyone who wants to continue this story to take on the personal responsibility of adding to it in the IC. In other words, I need you to "do shit." Try to write something that you know is going to give other players something to react to. Cause a problem, light a fire, be dramatic, get pissed. Don't only write reactive posts, but write things that make people want to get involved. You know what I mean? Create a plan, an agenda. What does your character want to accomplish? Which other characters do you think are interesting enough to get the attention of? 

Go do stuff.