You wanted to know why people left CN, and I'll respond with a question of my own - what more did we hope to get out of the game? What more did we hope to accomplish, and would it be worth the time & effort required to achieve it?
All the big names who have left would probably have seen everything the game had to offer. We'd won wars and we lost wars. We'd (hopefully) bounced back from defeats, and we'd been knocked down after victories. We'd probably done every type of work imaginable within an alliance. We'd optimized our nations, built to optimal infra, amassed warchests that would put Google to shame. If we were lucky, we found a tech and trade circle that didn't fall apart every month - and those who were really lucky found someone with five brothers to be part of it. We'd had countless arguments about dumb, inane things both within and outside our alliances, and seen the RL/CN line crossed far too many times. We'd stayed up until 1 AM just to hit F5 forever and then have 25 nukes blocked in a row.
So, I ask you - what more did the game have to offer? What else would we want to achieve, knowing that most things had already been done, and the things that hadn't would require far too great an investment of time and effort for even a minimal chance of succeeding? At a certain point, you know that you don't have any more goals or any more experiences to get out of the game, and logging in every 20 days becomes a futile exercise.
You also start to realise that, even if there were things you wanted to accomplish, your time spent on this game can no longer be a priority. Everyone is in the same boat - we're all a decade (or more) older than we were when this started - and our ability to commit the required effort to accomplish our goals reflects that, especially when there's a diminished payoff because no experience is new anymore. There may be a small hope that someone else will take up the mantle, but we lose interest if no one does.
In all honesty, I feel like you (ironically) summed this up quite nicely in your OP: