For me it was a few things.
The reasons I could articulate at the time were...
Obligation. I felt like you get married, you stay married. (Actually I didn't feel this way when I got married - I felt this way after I had kids). You suck it up and get through life, accept your situation, be happy with what you got, and make the best of it. You figure it out. You make it work.
Kids. I felt like I was throwing them under the bus if I left. I can see where that was not healthy thinking now but it was where I was at the time. The fear and guilt I had built up over this issue was enormous and paralizing.
The things I could not articulate but that I see in hindsight are:
I felt like it wasn't that bad and I was being a big baby. I didn't have a 'reason' to be so angry and unhappy so I didn't give myself permission to deal with it.
I did not have emotional boundaries. His pain, discomfort, suffering affected me very deeply in ways I did not understand. I would have said it was empathy at the time but it wasn't, it was lack of emotional boundaries. I didn't even recognize, identify, or ponder my own feelings. While there was total enmeshment in some ways there was very very rigid boundaries in other ways which prevented any kind of real trust or emotional intimacy. Despite the years we were together, family and life we shared, I did not feel a strong connection with him.
I knew I did not have that 'happy marriage' and it wasn't so much that I felt I wasn't worth it but I just felt like that was for other people, not me. It wasn't something I could wish for or expect and I accepted that I'd never have it so I had to figure out how to be happy with the way things were. I still feel that way actually and even though I can identfy that now and intellecutualize it away, it is still there on a feeling level.
The man I had in real life was not the man that lived in my head. That led to enormous confusion and vulnerability (to emotional abuse) on my part and even with hindsight I find it hard to believe that I was in that spot.
I still a long ways to go.