The text is not suggesting mothers cannot take care of ther children. In Afghan culture, that is what a woman's role is - to take care of and raise the children. Because this and pleasing her husband are what women are supposed to do, women are often illiterate and unable to work.
Moreover, even when a woman is literate and able to work, men are considered the 'breadwinners.' Women are culturally not supposed to be earning money and supporting their families, so people will not hire them. If mothers cannot get anyone to offer them work and hire them, they cannot earn money. Therefore, mothers are financially unable to take care of their children if the family is left without a father.
It is a cultural belief that women stay at home and raise the children and do not work. This has contributed to a severe problem in war-torn Afghaistan where there is a high number of orphans because many men are being killed through the unrest there.