As is Vermont (Ms. Magazine did a feature a few years ago about a woman there who service to DV victims revolved around driving them to safety, which is why I recall that its stat was about 80 percent of women were killed by a partner). Those are two that I know of directly.
A few factors are involved. A major issue is the isolation -- physical, emotional, and otherwise -- of victims. If you've got one car for the family in an area with no public transportation, and your husband drives the vehicle to work, and it's 10 miles to the next town over, how do you leave? In the case of the VT woman I mention above, that was the situation she faced in the rural mountains, and the reason why she does what she does for others today. Also, abusers tend to isolate women from their families and friends, so the victim may not have a support system nearby. Many, many places in this country have no battered spouses' shelters, no hotlines, no services to help.
And in some areas, the issue is cultural. Unfortunately, even if she does get out once, religion and socialization might tell a woman that even if he beats her, that's his right as a husband and her job to take it, that it really wasn't that bad, that he wouldn't have hit her without a good reason, and that leaving him is a sin. Many family members will send a victim back to her abuser, telling her she made her bed by marrying him, now she has to lie in it. Police might say the same (they did to a 17-year-old friend of mine with bruises in the shape of handprints on her neck).
Socialization is a huge issue in other ways. Men who grow up believing that women are inferior and that wives are there to be subservient are more likely to be control freaks and think it's acceptable, and even their responsibility, to hit their spouse. If law enforcement sees spousal abuse as a family matter, or there's a good ol' boy attitude, the abuser will know he can get away with it. Or if the couple are "pillars of the community" there's the idea that he would never hit his wife -- that only happens on the wrong side of the tracks.
A huge issue is financial. Domestic abusers actively work to make their victims dependent on them for money, especially if there are children. Often it's part of the isolation -- a woman has a child and stops working, at which point her spouse controls all the funds, forcing the wife to beg for money if groceries, diapers, or even medication are needed. There's no chance for her to save a little away, or even to control what is available. And, the cycle of abuse is such that the abuser poisons the victim's mind against herself, repeatedly telling her that she is stupid and useless, and would never survive without him. And if a woman is raised to believe that she's nothing without a man, that just reinforces his message.
In fact, the Justice Department directly attributes the decline in the number of women (as a percentage of the total population) murdered by their spouses over the past 50 years to the widespread entrance of women into the workforce.
These are just a few of the reasons for the geographic disparities
That said, a little known fact is that a woman is MORE likely to be killed in the weeks after leaving an abuser.
If you're interested in more info, try
this, the Justice Department's research arm.