I think that a lot of people get burnt out and take a break from the game for a while, which I myself have done several times, which reflects on the overall playercount. But I think a bigger problem for this DLC is the toxicity that has been slowly killing it, ever since it's start. Toxic people who have made the game less fun for others, I'm sure has been one of the main reasons that has discouraged several players from continuing to play NW, and further driving the playerbase down.
Oh, and also the lack of influx of new players. I don't think many people buy NW nowadays. Mount & Blade itself is quite an old game, so getting a new player is a rare occasion.