I think there are a number of factors at play here. I think a large part of it is that having children is extremely expensive. You need a bigger house, a bigger car, need to buy more food and then pay for their entire life for about 20 years or so. Given the cost to just sustain one’s self these days I think it is not a surprise that many people opt to have fewer children or none at all.
I think that also there is a culture shift that has taken place, there is less pressure to create a traditional family unit. More virtue is given to autonomy and following your dreams than settling down. In addition to this, contraceptives such as IUDs have become much more socially accepted, so less people are getting pregnant. Naturally all of this adds up to a drastically lower average fertility rate than a few decades ago.
In summary, it’s a bit of culture shift, economics and science which has led us to where we are today. The result is that in the next few decades we will either have to start having more children again, or rework the way our economies are structured to account for less growth in exchange for a focus on sustainability and providing a livable wage for all people. Its likely that if we keep with the status quo, quality of life will keep going down until we hit a breaking point.