Conversely, why would a husband stay with an alcoholic wife? I bring this up, because your question doesn't apply to just women. It means to me why would a normal person stay with alcoholic spouse?
Further, I think the answers are as complicated as the field of human psychology. I think a big factor, as is your first suggestion, that one always hopes the spouse will get better is probably a very common response. Marriage is an investment, and one always hopes it will succeed. And in fact, sometimes the spouse does get better, although statistically this is a bad bet. Recovery rates for alcoholics appear to be dismal. I don't know if anyone actually knows what the rates are, but I remember hearing somewhere around 5%. That sounds low to me, given that I associate with a lot of normies and former alcoholics, but if it's more than 5%, I personally wouldn't think it is higher than 15%.
Having said that, I'm guessing you're hoping your husband will get better. Other reasons obviously include marriages where both are alcoholics, and both parties get along suffering together. It could also be that the spouse provides an adequate income, which is important in a marriage, and while one hates the drinking, he/she also sees other advantages in the marriage. Another factor is just plain inertia; It's easier to remain married than face the big changes one has to make after a divorce. Some reasons make more sense than others, but the study of human relationships is complicated, because individuals vary so wildly in behaviors and motivations.
So are you seriously considering divorce? I think that's reasonable to consider it in your situation, but I wouldn't dare give you advice on what to do, especially with so little information.