Smart phones and the ubiquity of internet and video games is really it I think. You never have to be bored anymore, and therefore don’t have to meetup in person to be entertained.
So there was something smooth that hit a phase transition. Likely enployment or housing prices, forcing people to engage survival mode. People in this mode do not feel secure enough to have sex or especially start families.
Video games were available pre-2010, but they weren’t nearly as ubiquitous, and honestly the people playing lots of video games pre-2010 weren’t having lots of sex either.
Same with smartphones. Smartphone apps existed but weren't as personalized and didn't serve nearly as diverse of content as they do now. It's night and day. Surely I don't need to pull up a graph of smartphone usage per day.