It's two years since leafcutter ant update. People just don't have much to talk about to come back. When fire ant update hits, I'm sure many old timers and insane players will be active again. Deleting them is like saying: "Hey you guy aren't active enough so we don't want you here!"..., which is bad for a growing game.
I'd hate to be one of the guys who Kickstarted the game, helped test it, and is waiting for more content to come out to post only to see my account removed for some reason. Furthermore, I'd hate to be the community manager who'd have to deal with the fallout from that.