It is said that people do best when they know what to expect. I think there is a large amount of truth in that statement. Staying with your husband, you know what to expect, as bad as it is.
We also all wish to be loved, to have someone. Even though it once did, it does not meet that need now - but the memory will remain strong. Leaving him, you know neither what to expect and you leave what once was something that met your needs to feel loved, to have someone to love. It's a difficult step to take.