I’ll be crass and say that it probably has to do with people having less money to do the things that result in the cause of “unplanned” pregnancy. Fewer drinks over the course of an evening out, fewer cruises, fewer all-nighters at dance clubs, fewer chances to flirt on exotic beaches in the moonlight., etc. etc.
Maybe Sandra Fluke had a hand in this - since taxpayers will now have to pay for birth control pills and people are just waiting for this to take effect??