Maybe it's a combination of circumstantial things and the fact that we all have different experiences with drinking. I quit 2 years ago, and this is my first major attempt. For me personally, I was able to quit and stay quit because:
I was done drinking and sick and tired of what it did in my life and how it made me feel. It wasn't giving me as much as I put in, so to speak. No pleasure I could get from it was worth the pain. Others haven't gotten there yet and remember more of the "fun" aspects of drinking. It still provides a pull and an attraction. For me, I had gotten to the point where I felt that I was done.
My wife is a non drinker. I never had to face the temptation in the early days of sobriety of having to constantly see someone drinking around me on a regular basis, and having alcohol in the house. That was huge.
Everyone's experience is different, since we're all at different places with it. If I had tried to quit before I'd "hit rock bottom," maybe I would have been a serial relapser too. As it turned out, I made a full turn away from alcohol because it had soured my life, threatened my health, made depression worse, limited me in in many ways and had begun to cause crippling panic attacks. In the place of those things came clarity of mind, calm, peace, dissipation of a fog that poisoned my thoughts, a renewed self confidence, etc. Knowing a drink would knock away the latter and mark a return to the former, I've stayed away from alcohol.