In no particular order, not least because correlations are hard to tease out and frankly, the reasons aren’t all that great from a Christian perspective.
- Less sex and thus less pregnancy
- Morning after pill use tripling since 2002
- Condoms everywhere
- Less driving
Teens increasingly stay home and look sexy at each other instead of Fast Times at Ridgemont High (which was fairly realistic for the time and correspondingly ugly and depressing). When they do manage to fornicate, they try really hard to remember to use their STD/birth control and if they forget, they race for the morning after pill.
During the 70s and 80s, that was when abortion was the big thing causing (mild) decreases in teen pregnancy, but that hasn’t been a major factor in the recent declines.