Previously it was a good idea to have lots of children, because they were useful as labor force for the family at countryside. Moreover, culturally speaking, it was a signal of fertility and masculinity for men to have many children. However, nowadays people are much more focused on themselves, caring little with everyone else, what includes potential children.

It's a phenomenon of our days where people are extremely egoistical and selfish. As consequence, there is little room for children... In previous times people weren't even concerned if they had enough financial conditions to raise a family, but they always managed to do so somehow.

Another possibility is that people had stronger bounds before, what allowed them to have genuine desire for beginning a family. Nowadays people get bored of each other too fast and bounds aren't for real.