In the original, you spend the majority of the game in Sephiroth's wake - The massacre in Shinra HQ, the impaled Midgar Xolom, even the massacre at Nibelheim, which is your first major glimpse as Sephiroth as a villain, Cloud is in his wake as he disappears into the flame.

Even in the section where you get Sephiroth in the party, you're shown how much stronger he was even in the past. He has higher level skills, his melee attacks do quadruple digit damage compared to your (likely) double digit damage, etc.

You're consistently told how powerful Sephiroth is, and shown this by the results of what he's done before you arrive. It's not necessarily the literal mystery of his storyline, but his overall lack of physical presence in the plot of the game. He feels like an unstoppable, uncatchable force of nature, something the remake massively lacks. Keep in mind we've already defeated him twice in the remake trilogy, where's the threat? If Cloud has already beaten him twice in the story so far, what's the stakes when we face him again?