What changed is perspectives. In communism you'd earn almost the same no matter where, how much, and how well you worked. And money mattered little anyway, you couldn't buy anything with them, you had to have connections to "arrange" anything (even basic building materials were "arranged" through social networks not bought from a shop). So people invested in families and big social networks to survive. And had lots of kids cause there wasn't much else to do. No career to sacrifice, no recreation other than drinking and partying. No internet. TV had 2 channels and most people had only black-and-white receivers. Culture was only accessible in big cities. Half the families had no car. You couldn't travel abroad easily. There was censorship. There were blackouts at the night.
Why not have kids in these circumstances? What are you losing?
Now your standard of living depends mostly on your education and working ethic. And it can vary greatly. Sky is the limit. You can have a yacht. You can travel all over the world. You can be unemployed and live under the bridge. And anything in between.
You don't need other people to survive. Money are very important. Career is an option. There's LOTS of ways to spend time and money. Having a kid is a big sacrifice in this world. So people work more and have smaller families.
The parts of each country that are less wealthy are usually also the parts with highest natural growth.
Basically - economic growth and development reduces natural growth. Pro-family laws are nice, but they don't change the basic calculation of pros vs cons as much as the changes in societies did.