The Lessons Not Learned.

[Absimiliard]

That's generally the impression given when an FF9 fan tries to claim it was more popular than FF8 or 10.

Also, isn't FF8 also considered unpopular due to its gameplay and confusing plot past CD1?

FF8 was received well by critics and had high initial sales, but it lacked staying power. Much of its early success is noted to have been a direct result of Final Fantasy 7 having left such a good impression on players. The junction system was the aspect of the game to receive the most criticism. The handling of Guardian Forces (summons, for those who might read this and haven't played the game) wasn't overly well liked either. The game's a little more fondly remembered these days.

FF9 sold far less than Final Fantasy 8 both initially and overall, but much like FF8's success was due in large part to FF7, a portion of FF9's failure to move copies was attributed to the impression left by Final Fantasy 8. Much of FF9's criticisms were aimed at its weird discount junction system as well as how new skills (and summons) were learned. People didn't particularly like Trance at the time either. The few fans aware of it at the time also heavily disliked the fact getting the best weapon in the game (Excalibur II) required you to sacrifice pretty much everything else.