1) Shifting cultural norms. People are now enough and they want to live their lives. Kids are annoying as hell to be honest, and until you're a parent, you don't get the appeal. But nature has a way of really playing your heart when you have a kid and the intensity of emotion is hard to describe. BUt if you were to do a pros/cons list, you probably would not want them.

2) Inflation of expectation on parenting. Parents used to be much more hands off in the previous generations, but the expectation of what you need to provide has gone thorough the roof and not all is useful, to be honest.

3) Delay in child bearing led to fertility drop when people really want to have kids.

4) This, I somewhere and will pawn it as my own, but... kids are no longer useful. Previously, kids were a working hand around the house and insurance for parental care when they got older. Both of these are not something necessarily expected of them nowadays.

5) Since women bear the brunt of the rough jobs of child rearing, even in our more egalitarian societies, their increase in options and freedom means a lot opt to just... not do it. As mentioned at pct1. it's really a tough job. Can't say I blame them.