It’s always been like that in FF14. People come for the new story, level up their class, try the new raid. Than take a break until next patch. Square Enix never rushed any content they give time to people. Only the most dedicated (Those who play craft / RP) or those who pay land taxe (FF14 house owner) remain to the game.
Things will pick up more when they release the Alliance raid, and some of the newer miscellaneous content. The Dev presented the new Expansion road map, looks pretty interesting and there’s a lot of activities to come that should create some retention for a while when it happens.