For one thing, the graphics were cutting edge when they were new. Granted that some of the elements aren't amazing for the second decade of the 21st century, but almost 30 years ago they were absolutely bitchin.

The combat systems often had something experimental about them, whether they worked or not--junctions and ATB in VIII, for instance, with guardian forces, limit breaks, etc. It was apex JRPG and the gold standard for a long time. Once again, not everything worked for everyone, but you can't please all the people all the time.

It also helps that the characters and worlds were unique and new in every game.

I'm sure others will pipe up with their reasons, but those are my opinions on questions you asked.