On the other hand, while I can think of a great many single mothers in fiction, I can think of very few stories in which a woman's actions are primarily motivated by her husband's death and her struggle to come to terms with her loss.
Why is that, do you suppose?
I somehow doubt that women are less affected by the death of their husbands than men are by the death of their wives. I'm guessing the reason has more to do with reader assumptions. I suspect that most mainstream audiences are willing to accept the idea that women (or at least a certain type of female character) are not always rational. Thus, if your plot requires that a female protagonist behave irrationally, it is not necessary to kill her husband to convince the audience to accept her behavior. If you want a man to act irrationally, however, or if you want him to struggle with powerful emotions, audiences are likely to view him as whiney or unrealistic unless you provide a very solid reason for his behavior.
The truth or error of these stereotypes is a topic for a different debate, but given that the stereotypes are present, the predominance of widowers in fiction begins to make sense. In my opinion, it has very little to do with the actual condition of the world, and a great deal to do with the preconceptions of the audience.
I think I am going to be cautious of using this plot device in the future.