What's going on? There's some free-form speculation in the original post, and in that spirit I'll offer some of my own. I'll bet that part of the overall decline has to do with improved medical care: husbands and wives are still trying to kill each other, but the advent of universal 911 and better trauma care means that a lot more people survive these attempts. So then the question becomes: why are men surviving murder attempts better than women?
Perhaps women are just less skilled murderers than men? Perhaps women who try to kill their husbands are more likely to feel immediate remorse and call 911? Maybe it has to do with choice of murder weapons. I dunno. But I wouldn't be surprised if differential survival rates are part of the story here.