FF7 is not a great FF game; in fact I wouldn't put it among my top five (6/3, 4/2, 8, 9, 10). It was very cookie-cutter, with the exception of it's presentation, which was a brand new concept.
The reason for its popularity is best described by analogy.
The first time you have sex, it surely isn't the best you'll have in your life (probably among the most awkward and retrospectively shitty attempts), but it leaves an impact. You'll never forget that first time.
Also, we're talking about a game that defined RPGs for the new generation. Most of the people that gush about its greatness were new to gaming at that point, or new to RPGs. They were young for the most part, and part of the generation that truly grew up with the internet and spew their intellect (or lack thereof) across message boards. And they don't shut up, either. Hence all the hooplah for a generally average RPG.