They do have a lot of projects going on within just the Final Fantasy franchise (VII remake, on-going of XIV updates, XV, and Explorer in the West), on top of that, from a marketing point of view, I personally don't think diverting the focus to yet another series is a good idea at the moment. VII is a bit of a different story as it's well established to be the fan favourite. I can imagine after the release of Explorer in a week or so, they will focus their entire marketing effort into XV. XII might be in the works, but I think we just need to "please wait a little longer."