While videogame interest dramatically increased, interest in Square and Final Fantasy games generally stagnated (they went from obvious hot **** borderline inescapable monopoly of the genre to just another company in that time)-- the two things more or less counter each other, in my mind. I really don't think it plays that much of a factor when accounting for IX underperforming, especially considering how much it did compared to everything along that timeline, not just the newest games.
Whether we want to call it weeb sensibilities or not, I definitely think the fact that some of the games are designed to be packed with conventionally attractive, easily-marketable idol-esque characters and some of the games aren't/don't played the biggest factor in their marketability (given that production values relative to their time and brand recognition was high across the board, anyways)-- games that have a distinct and eccentric style/character to the way they look tend to become very niche and limited (many of the primary characters that would be heavily relied upon in marketing campaigns, such as Zidane and Steiner look unsettling and like borderline ugly people to most), whereas games where everyone looks like a supermodel are potentially highly marketable to the masses, even when they're not very good (I mean, XV leaned HEAVILY on that and everyone makes fun of it for that reason). I don't expect a guy like Matsuno's work to ever become all that commercially huge, for that reason, whereas a guy like Nomura always exploits that aspect and is rewarded for it.
It's not like the sales figures of the franchise go up and up as videogames became more popular or something. IX dramatically underperformed compared to VIII just like XII underperformed compared to XIII or XV. I mean, look at the character designs in VIII, X, XIII, and XV compared to IX and XII. There's a very clear trend there in terms of broadness of appeal.
Hell, while obviously there were a ton of reasons for FFVII becoming a cultural phenomenon, I doubt that it could have been to nearly the same degree if Amano (even though he's waaayy more talented, artistic, and brilliant) remained the lead character designer for all the promotional material rather than Nomura and his Anime pretty-boy designs.