Because at some point down the line, there will be many, many stay at home moms and dads who will have no means with which to build their own individual credit histories. When those people become divorced or widowed (or whatever circumstance might befall them), they will be woefully unprepared to conduct their daily financial chores.

On the one hand, I can see the sense of making the one with the income be responsible for the debt. But on the other hand, there are scores of traditional family units where one spouse raises the kids full time. If those households pay their bills on time and manage their credit wisely, does it not reflect on both of them as partners? Doesn't the stay at home spouse get any credit for running a household with some financial discipline?