Not a historian, but I'm guessing the primary reason is because for much of human history, two-parent families had one parent dedicated to raising children.

Also some other reasons:

1. Probably there was more general family help with children - grandparents/town/etc was more involved.

2. For kids that were a bit older, they were effectively part of the workforce - apprenticeships, farmhands, etc.

3. Probably, it was more safe to let kids wander around in many cases - cities are particularly complicated, what with streets full of cars and just generally unsafe areas for kids. In a small town, or a family farm, you could let toddlers run around a lot more.