It is arguably the 2nd most successful MMORPG of all time outside of WoW. And people stuck to this game for so long but why? And why don't people learn from FFXI, another successful MMORPG and an alternate way of presenting content than what you see in WoW.
Namely, the need to interact with others to meaningfully gain progress vibrates throughout all parts of the game.
From leveling, to questing, to raiding, to PVP. This created the strongest sense of community I've ever experienced within an MMORPG. And as far as I can tell, FFXI is unique in a sea of MMORPGs in this way because no one has ever attempted this style of gameplay again.