Because those are popular, and Minecraft isn't finished development, there's still new content being added because Mojang believes there's more to be added. Fortnite BR is still receiving content because it's an online game with servers running constantly, when it inevitably shuts down, it'll die out almost immediately because no one will be able to play the game anymore. Terraria is no longer receiving content because the developers are proud of their work, and believe there is nothing more to add.