Not sure I can speak for people born after it released, but I played it around when it came out. I was young and it was enjoyable for me for many reasons. The materia system is simpler and still more enjoyable than most other systems. There was a good air of mystery while still explaining the world itself in great detail. Unreliable narrator is a strong storytelling tool. There was an obvious enemy from the start to get you invested and yet it still shifted as you played. Some other games you don't really know why you're doing anything for 20 hours.

FF6 was just before the time of big advertising pushes and is dated graphically for some people (I know people who would try 7 but not 6. sounds funny nowadays but my anecdote). FF8 story felt silly but had a cool main characters with bad combat. FF9 was cartoony with the world under explained but excellent characters. FF10 got about the same reaction from me as a kid as 7 and I think it also has a pretty loyal fandom for similar reasons. It also has the great sphere grid system and was fun to play and mess around with mixtures (like 7 was). FF12 license system was pretty awful and the game suffered from pacing and Vaan.

People want a cool main character. People want a twist (or 3). People like emotional impact of losing someone or finding out they were lied to. People like deeper feeling worlds. 7 hit on most of the important stuff.

FFVII was many gamer's first rpg ever

Was also a big part. It got the most advertising budget of any game so more people tried it. It was good enough that those people spread it pretty regularly word of mouth. So if you were trying to get into RPGs it was often recommended by someone.

My rambling 2 cents.