Yes probably the reason it was the best selling rpg is because it was the first final fantasy game released on the playstation.

Before then rpg's were not extremely appealing to many folks because of the graphics. And though you might say that graphics don't matter but in a sense that it makes it easier for the player to get into the characters of the game and the story of the game it does matter.

Add to that is that they really made an effort to develop each and every character to detail. You really got a sense that you knew each character by the end of the game. And to polish everything off it had a very memorable soundtrack of memorable character themes that helped the player get even more into the games characters.

Those are the reasons why I believe it turned out to be the most popular final fantasy game made up to this point.