Because it was a fantastic game.
In my opinion, what makes a great RPG (whether it's a JRPG or a WRPG or any type of RPG at all) is the characters. Final Fantasy 6 had the best characters out of any of the games in the series. And each character had their own abilities or special uses, so no character was truly useless (*cough* Cait Sith *cough*). Additionally, Kefka was an incredible villain. There was nothing tragic or emotional about him, he was just a cruel, heartless bastard who wanted power. Combine that with a plot that has a lot of interesting twists and turns, and you have one of the best Final Fantasy games (if not the best).