I haven't reaqd everything, but I think a HUGE reason why many families can't get by on one income is due to the fact that so many families have divorce/support issues whereby one person isn't supporting 1 family, but 2 or 3 or even more families....
for example, when I met dh, he had 2 seperate previous families he was paying support to..his first ex-wife for his 2 kids with her, and his third exwife, for his 3 kids with her.
Now, of cours,e this is all based on choices and things that happen, but quite frankly, the vast majority of women my age (30s) that I know, have a spouse who is paying child support, to either a previous wife or "babymama", such that his 1 income is supporting more than just 1 household. On the other end, of the women I know have money coming into their household fom someone other than their dh.... most of their dh's are paying out support as well, so at best it tends to be a wash, but usually it seems like they get less in than the dh is required to pay out...so you have these "families" where there is money coming in and going out all over the place, but the old concept of one single, nuclear family is a lot less common.
for a long time, we joked that the best way to make our family financially stable would be to get me knocked up by someone other than DH, such that we could get some child support rolling into our household budget. Crass, but true.
Also, families such as mine often aren't eligible for any help/benefits, because programs look at gross income before deductions. SOm even though yoiur family income and mine might be equal in actuality, my family income looks a lot bigger on paper, because it includes the money dh pays out in support that our family never even sees, so *my* kids aren't eligible for any of the help that so many families take for granted ( state health insurance, wic, reduced lunch at school, etc) because on paper, it looks like we are a family of 4 making $40K a year...when in reality, it is a family of 4+3(half the time) making $30K a year.
Anyway..I really think the fact that society has changed in terms of the composition of families, the huge rise in divorces, and the child support/alimony issues, are all serious issues to look at, that play into the changes just as much as other factors.