Because our 3 main characters (Zidane, Garnet, Vivi) looks like kids. The adults don't really look like adults. It's more of a chibi art direction, but the general impression of Final Fantasy to people is for it to be 'realistic' and 'dark', thanks to FFVII and FFVIII.

So when FFIX comes out with a more cute direction in both story and art, people tend to wave it off. If FFIX was renamed into something else, it might be better off in terms of general public critism, but FFIX, even with the "FF" at the front, is only a cult hit. Square might not make the sales it wants if it was renamed. They'd have to do alot of marketing for that. I guess they are just expecting the name "FF" to kick off their sales =/