We now value professional careers above everything else. You can't have kids and be competitive anymore. Half of the people I know who have kids basically delegates everything to caretakers, they see them 30 min in the morning, and 30 min when they get back to work and kiss them goodnight.
Instead of telling men to slow down and take care of their kids we tell women to forget about kids and follow men in their insane quest for professional conquests. Sprinkle a bit of consumerism over that and it's game over, kids are too expensive, why would people do that instead of buying a nice car, the new $2k smartphone or a trip to Venice to flex on instagram for imaginary internet points.
I'm also inclined to believe that the hookup culture plays a huge role in the trend, why would you want to settle down when you have a virtually limitless pool of potentially better mates available at the tips of your fingers. Newer generations are afraid of commitment and responsibilities, having kids in your early 20s is more stigmatised than having a one night stand every other night until you're 40.