First and foremost, up until the NPD bomb I was still hanging onto the idea that things would get better. I firmly believed that given enough time and the right atmosphere, W would come around. If I was patient and steady, W would eventually feel safe enough to open up - both emotionally and sexually.
Apart from that, my religious beliefs and the values inculcated in me as I was growing up tell me that staying with W is the "right thing" to do. As you've no doubt heard me say before, I put an extremely high value on doing what's right. That belief in doing what's right manifested itself in several areas.
For one, I've always been one to put the needs and desires of others ahead of my own. IOW, I come last. I know that I sometimes carry that to an unhealthy extreme, but you asked for reasons without any requirement that they be healthy and sane reasons.
After the things my kids told me after the NPD diagnosis, I was probably wrong on this one too, but I also placed a very high value on raising my kids in an intact family. I now realize that I was blinding myself to the damage that W was doing, but like I said about the M in general, I was still hanging onto the belief that things would get better. As before, you didn't mention any requirement that the reasons be justifiable.
"For better or for worse" includes for worse. I made the commitment and was bound to it. This was also tied in closely with the Biblical injunction not to deal falsely with the wife of your youth.
Now, after learning about the NPD and the prognosis, I'm still around for a couple of reasons - primarily the many facets of the "right thing" argument. And if I'm totally honest, inertia is a big reason too. It's been this way for over 31 years. It may not be good, but I've gotten used to it and there's something to be said for comfortable.