The key reasons have been mentioned. Cost, housing, scary world and social media.
Where it was a norm to get married and have kids. You can now do and be whatever you want.
I think the emphasis on individualism plays a role.
I have 2 children - both surprises. Had one young when I was naive and managed to work my way up my career but I had a supportive “village” - so many people just don’t have the support. Plus it’s acceptable to not want them - and if you do want them then have them.
Children suite my lifestyle - and I think people are more conscious of if children fit theirs (which is actually a good thing) - I’m super homey - work for myself - enjoy parenting and don’t care for crazy global adventures - but if I didn’t fall pregnant I would delay having children as much as possible - simply because there is so much more you can do with life but also it harder to get the basics like a house, climb up for a decent salary etc.
Lots of my friends held off having children until being older (I don’t blame them). But now that they are finding their long term partners, issues around fertility has been a huge concern with my friendship group.