Because the drop rates are very low like many korean MMO's, so there's lots of value in continuous play,
you can upgrade all of your equipment to +10 by using enhancement items that can be found or purchased for a high amount,
theres items that must be unlocked with other rare items (before they become powerful),
theres pvp,
you can fully customize both your stats, as well as your skills,
you have to find your skill books on the map (many rare from certain mobs/bosses),
basically this game's for the elitist.
You can't beat the end game bosses by yourself - you need a group, the bosses cannot be kited outside of their their general area, or else they will reset,
The bosses have many unique spells, that if you can possibly slay them, they have a chance at dropping,
The crafting system for a lot of the equipment is cool, especially becuase you have to find many of the ingredients, materials, and the recipe itself in order to create certain items.
I haven't played it for a few years, but just off the top of my head...this is why I, and many others enjoyed it.