I don't believe the men are actually 'forced out' or mysteriously disappear. This line (I believe) refers to Gentlemen in the social life of the class focused on in Cranford. These men do not then actually have to be gone to have 'disappered' they just have to be socially inactive or unimportant.
As most of the women are elderly it is likely that most of the men have died off (naturally) as at that time it was quite common for men to marry younger women (and so they often passed away before their spouses). If meomory serves me Cranford is a sort of retirement area, few young people (of the class focused on) live there and so this explanation seems the most likely to me, especially as time stretchs in Cranford and the men may take a year or more to 'disapper', and still be thought to do so. They may also simply suffer from some age-related illness and so not 'go out' and thus not be seen in social life.
Added to this, as I understand it, the women simply dominate. The men in the town do not really participate in social life because it is a women dominated area and the younger ones probably do soon leave to find better prospects. The older healthy Gentlemen probably go off to their club, or stay with friends frequently because their is not much social life for them in Cranford and so are not seen much. While their wives are seen because they do enjoy the social life in Cranford.