It seems strange to me that this is the case in a land where the by-far biggest successes of the genre continue to be very much the series they were when they began 10+ (Pokémon) and 20+ (Dragon Quest) years ago. But clearly things don't seem to be panning out for 4WOL. Perhaps it is that they've got enough legitimately steeped-in-history series that there's not room for a faux-retro game that's not actually a continuation?
The problem with FF is that with their various spinoffs and new installments they have lost for the most part that sense of branding. There is no connection to have with 4WoL in terms of being a FF fan, so it can only be approached from the direction of being a JRPG fan. This also limits it's sales potential significantly. The game has no chocobos, no moogles, no summons, no character design similarities to any other FF, etc.
As far as marketing and branding goes, 4WoL is about as "FF" to the general public as The Spirits Within was. It's FF in name only. On the other hand, a title like FFV DS or FFTA3, would be not only easily seen as an actual FF branding title, but also be part of an established franchise which already has a larger fanbase waiting to eat it up.