It’s really simple: kids are a chore. Once you remove the economic and social pressure to have kids most people will opt out of the experience and do more interesting things instead. In developing countries economic pressure exists (kids support parents when they are old) but in developed countries only social pressure is left. Once it becomes socially acceptable not to have kids people just stop. In some countries like France kids are still a status symbol so people try to have 2 kids. In other countries kids are used mostly for socializing: when all your friends become parents it’s hard to hang out with them without kids because all gatherings become kid parties. Most couple have one kid because that’s enough to be included. But as more people don’t have kids it’s easier to find friends without kids to do things together and another motivation is gone. There was also a pretty big shift in how people approach childless couples. Asking “when will you have kids” is becoming a faux pas because people are more aware of fertility issues so another source of social pressure is gone. The truth is that even in perfect financial situation few people will have more then 2 kids (just look at the stats). In less than perfect situation people decide to have 1 or 0.