It was a year ago, perhaps more, when I filled out a survey for Square-Enix about my favorite Final Fantasy games. As I play through Record Keeper, I noticed that they started out with a scene from 7, then dipped into some other games, but after getting a taste of most of the games (nothing from 3, 8, 9 or anything post 10 just yet), there has been an intensely strong focus on 6 for quite some time now. Three of the named characters I have are from 6, others are one-ofs, save 4 from which I have Kain and young Rydia. Terra and Celes were from special events, only Cyan came from simply playing, but there have been so many more story events for 6 than any of the other games so far.
I have to imagine they are holding back on 7 (and started with it) since it seems to be beloved. Keep the players wanting. But this heavy focus on 6 simply has to be because of some survey or something tipping off SE to the love the fans have for 6. At this point in a Free-to-Play game it can start to feel real grind heavy, but to give the player a nostalgia trip right then? That has to be calculated like so much of these games is.