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...
You say people are egotistical and selfish to not have kids meanwhile at the start of your comment, you mention, having children was good idea so they could be used as working force — you realize how absurd that sounds?
Why absurd?
That was one of the main purposes people had lots of children in ancient times. People lived in countryside and needed abundant workforce to plant, gather and to feed the animals. There weren't automatic procedures to make it for them. Then within time, the number of children per family have been decreasing, until the point nowadays there are many people who don't even want to have a single child.
If it wasn't for this concept of family, probably those people would starve, unable to keep all the daily tasks completed. Then there wouldn't be anyone here today to claim how absurd that was...