FFVII just set a staple for the kind of quality you can expect from an FF game. A complex and interesting story with exciting and relatable characters, backed up by breathtaking music at times. (Ok, I know FFVI, FFIV and others probably did that but not many people played 'em). It then introduced beautiful 3D FMVs in a 3D environment, which is the reason why it got noticed.

The FFs that followed are not neccessarily any worse, its just that people aren't as blown away anymore. They have expectations of Final Fantasy which they didnt have at first. And also theres the phenomenon of the FF experience. Your first FF is often your favourite. And FFVII was many people's first.