The “because social media” reason holds no weight, because social media had already taken the place of message boards/forums in the minds of most by the time Fates released, and even moreso by Three Houses, but Serenes Forest was still active then. At its peak activity, even.
Rather, I think a more reasonable explanation would be that Fire Emblem is no longer a thing popular with the general public, and has gone back to being something only diehard fans have stuck with. Fire Emblem is still more popular than it was pre-Awakening for sure, but it can’t be denied that the days of 2019 are over.