The big problem that UO ran into was that it turned out the people who liked what UO was is a pretty niche audience. In a lot of ways Everquest was a direct rejection of the features that folks like me think of as the golden years.
But to answer your question, there are three different clusters but contradictory sets of answers. And this was the problem.
1) It was a sandbox game developed with a focus on recreate a living world. A real ecology, real economy, skill based character system instead of classes where your skills tracked what you actually did, a focus on all sorts of roles - part of the original pitch was players could be the town blacksmith or whatever. I knew someone who spent several months playing an interior decorator for instance. Some people, such as myself, were attracted to this.
2) The same freedoms from #1 attracted PvP style gamers, especially from the then nascent FPS style games. Griefing, rampant slaughter, that sort of thing.
3) It also attracted PvE players who weren't at all interested in a realistic world and demanded the sort of conveniences we see in modern MMOs: mobs pinned to locations, predictable drops, predictable quest lines, instancing, optional PVP, etc.
You'll note that most of the people you see reminiscing online are from groups #1 and #2. Group #3 by and large hated the game and left as soon as they could. And your typical group #1 player eventually got annoyed at group #2 and just left altogether.
It's a hard problem to recreate UO because of this tension. Without allowing group #2 to exist you don't have the same environment. But by allowing group #2 to exist, they'll eventually take over and chase away everyone else.
At the end of the day, UO was a game that was simply a moment in time that can never be recreated. Too much of what made it great was due to the fact that it was a new thing.