I'm not entirely sure that's the case. When I briefly looked into the community for this game when I tried it, a hell of a lot of the people talking about it had no interest in the main series FF or even FFXV itself and had never played either. My perception was that there were as many general mobile gamers as Final Fantasy fans playing it.

Square pushed the game incredibly hard through things like BlueStacks (an Android emulator) offering rewards through BlueStacks for playing it for example and I think it picked up followers that way.

Make of that what you will!