There are a lot of maxed Divination players because the community has declined in number and is now more disproportionately dominated by veterans and older players. The rate of newer players joining is abysmal, as is the number of free players and newbies. Also, the culture of efficiency above all has been firmly entrenched. Everyone is interested in getting levels and less interested in fun and 'time-wasting'. All of this adds up to lots of people getting 99 Divination very fast despite it being a slow skill, by means of probably fiercely grinding.
How does "not getting new members" have anything to do with a higher number of maxed divination players? Your statement "the rate of newer players joining is abysmal" seems to be accurate, but doesn't actually have any bearing on how many people get 99 divination unless you're saying that new players joining the game would force old players to leave?
So I think the most accurate answer to the high number of 99 divination players is that the game has been around so long that the knowledge of the mechanics of the game is at an all time high, and the obsession with efficiency likewise is at an all time high. Both of these work toegether in contributing to players with 99 divination. Also there were different motivating factors for getting 99 this time around. Before divination came out, there were so many players that were already maxed that "lost their cape" or lost their prestige or w/e. I believe that this would have added motivation for a number of players to grind to 99.