Everquest was just as addictive as WoW. In many was EQ and Final Fantasy XI were both more addictive and more immersive than WoW, yet WoW's success dwarfs them both. It can't be the only explanation.
With EQ, big rewards were frequently too far in between (especially hell levels and gaps between major "new spell" levels), which led to people quitting when their addiction waned (ie, minor rewards were considered rote, major rewards non-existent or too far away). Also, the end game (at least until Velious) wasn't nearly as developed as WoW's is, and relied on raids held by huge guilds or even the entire server. This led to higher-level players frequently not having access to any near-term rewards, and eventually losing the addiction.
FFXI's problem was with lower levels. The reward for gaining a level didn't seem as important as in either EQ or WoW. Equipment never seemed like a major reward either, as most players essentially rented equipment from the auction houses.