As well as the obvious financial stuff…
An ever growing cultural focus on the individual and doing what makes you happy, far more alternatives for what to do with your life (and visibility of alternatives through social media etc) and what might be called “selfishness” though I don’t mean that in necessarily a bad way. I know a couple of women of my parents generation who had kids because that was what you did, but who say if they were thirty-something now, with the opportunities they now see and with far better contraception, they’d never have had them. Certainly not 3/4/5.
Also the level of parenting expected now - I don’t have the emotional or physical energy for parenting a big family to today’s expectations. Parenting and children have become all encompassing. But I could have parented four the way my parents did in the 1980s - far less emotional input, far less child centric holidays or weekends, very few afterschool activities, kids piled into cars without seatbelts each in big carpool arrangements, we were allowed out unsupervised for hours, left in cars alone - my parents weren’t bad parents and we had a happy childhood but some of the things they did would get a safeguarding referral these days!