I wonder if there is any correlation in the numbers equation with when marketing (commercials) started and slowed, or ended. I personally don't see commercials anymore. I would guess, exposure depends on what kind of channels you watch. But I would think it would be a factor. If a channel they advertised on was losing viewers that would have an effect too. Little to no marketing, no retaining new customers to counter balance those who leave for their reasons. Ya go bald when you lose more hair then new growth.
Other factors would include economy peaks and valleys. Take note: The "Great Recession" of 2008, refer to Lusche's number charts (a great informative response). I would think, things like computer or gear goes bad and no money to replace it is a factor. Which was one of my reasons I left. Time and new gaming pc is why I returned, because of better economy. I never left because the game was bad. Obviously, now common on the boards, many are returning for the same reasons. They probably left around the same time too, another common theme. Despite inflation (inflation exist because people have the money to spend too much) the economy is good, people buying new and much better gear, this is prime time to be advertising, even just a little.
One point being, the game is better off now then during the peak. Marketing could have increased to get the word out to new players.
There are a lot of variables in why numbers are what they are in all businesses. From all the readings in the forum I did, going back to when I left in 09, there were some common themes and dates. When looking at his charts, my GUESS would be that a couple small unfortunate things happened, almost at at once, that had an impact. Which have nothing to do with the quality of the game.
2 cents