I think I stayed for a whole host of reasons... of which evolved with the evolution of my codependency.
I stayed initially because I was niave to alcoholism. I believed the "I'll settle down when..." stories. I believed he would "outgrow" the drinking when we got married - so I married him and bought a house together.
The drinking didn't stop... but I stay because I took a vow - "in sickness, and in health - til death do us part!" I did go to Al-anon - but I got my recovery all screwed up and mistook the "focus on me" for "I am the problem." So I stayed because I believed if I contort myself into the perfect wife - our marriage will get better and the drinking will go away.
So... basically my reasons boiled down to my denial.
When I came out of my denial - and truly accepted that it was alcoholism (and abuse) that I was up against - and I admitted my powerlessness - and I found boundaries and determined what I wanted FOR ME....
I got the strength to let go and leave. Al-anon (the second round!) got me out of my denial.