Majin-Tenshinhan on 22 December 2009
Let me start off by saying that I can't back this up with proof, but I severely doubt that FF7 was "the first 3D RPG", as you put it. I doubt it was even in the first 100 3D RPGs.
Either way, I think FF7 got off the ground because it got good advertising and was backed heavily, so it was pretty much the beginning of the series outside of Japan. By this I mean, for a lot of people FF7 was more like "FF1". And as with most series, sequels rarely reach the point of their predecessor. Some of the people who bought FF7 just didn't want to take part in the series anymore. But a lot of people did, which shows why other FFs have had strong sales.
