People had a lot of kids back then because they tended to "add value" to the family enterprise -- whether piece work or farming. Plus, kids tended to do off at a pretty regular rate.
Today, kids don't add very much value and parents tend to have as many as they can afford, which means insure they receive adequate survival skills/tools in the form of an education.
True, and also people had lots of kids because many more were likely to die as babies and children.