It was so popular because of how difficult and rewarding it was. There was none of the immediate gratification that plagues all MMOs nowadays. Much like tribes, it had a high skill curve and there were significant differences between people that play an hour a day and those that played eight. The original game had the very real danger of dying deep in a dungeon and not being able to retrieve your body and loosing everything you've worked for.
I remember when the level cap was 60 and leveling from 59-60 took weeks of hard grinding. Killing stuff in EQ, even in a group setting, requires coordination and skill... Not 5 seconds of spam clicking per mob like WOW and every other mmo since