The shrinking popularity of this forum is not caused by this forum, it's a consequence of the evolving Minecraft ecosystem. At this forum's popularity peak the way people played Minecraft differed greatly from the way that it is played now, the role websites played in the Minecraft ecosystem differed greatly too. As the game has evolved, so has the ecosystem. You can see this reflected in the traffic of this website, the Minecraft Wiki, and many other major Minecraft websites. The fall in traffic here is not unique to this forum. Furthermore, it's important to understand that discussion about Minecraft has always been a minority here, this forum was primarily driven by players looking for content and some of those people stuck around to discuss Minecraft. Fewer people here for content means fewer people here to discuss the game, and those that do stick around have less and less to talk about.
At the peak of Minecraft's popularity amongst PC players, the game was very different: a player needed this forum and a player needed the wiki to be able to play the game. Since then the ecosystem has evolved substantially, players are not dependent on this forum or the wiki anymore.
The way that console players exist within the wider gaming community differs from the way PC players exist within the gaming community too, check out any website focused towards console games, and even the most successful have very little in the way of forum discussion: that's just not what console players want. For example, futhead (another Curse website) is a website about the console game FIFA. The website's traffic is approaching an order of magnitude more than the traffic to this website, yet their forum is less popular than ours.