It may just be as simple as more people don't have kids these days because in our connected world people have a greater realization of how challenging raising kids can be. People gravitate toward the negative in information so they see things across all the various information mediums of the challenge financially, emotionally, time-wise, the responsibility etc, and that resonates a lot more than anybody like me saying while it is and has been those things it has also been extremely rewarding, fulfilling and wouldn't trade it for the world.
People can easily attach to more concrete notions of "I'm gonna have less money, less time, emotional stress, and more responsibility." It's harder to attach to the vague notions of "rewarding, fulfilling, etc". Less money is less money...concrete, factual. Fulfilling is an idea that varies from person to person. Are you gonna risk what you are pretty damn certain will be the case (less money) for something you can't be certain of until you get into it (fulfilling)?
I dunno...just spitballing.