Because the world shifted to the right a bunch more regarding sexual content… fucking websites now want you to show your ID in some states to view porn.
I think acclaimed director Paul Verhoeven put it best when he called our country a land full of gleefully violent prudes when people freaked out about his use of sexuality, nudity and people being comfortable naked around members of the opposite sex.
i think it was prevalent before the interwebs because there was largely few places to get porn, and throwing it in a movie meant more eyeballs.
as porn became immediately available in other forms (mostly the internet), the unnecessary scenes could be eliminated as a waste of time and a detraction from plots. they ceased being a reason to draw eyeballs.
You can get gratuitous violence on the internet, too. Far more than the most violent slasher film. Availability isn’t the reason.
I wish the graph went back another twenty years so we could see whether the timeframe matches your hypothesis.
Bingo. This is 100% the reason, and it’s funny that people assume anything else. When you can just watch porn at any time with no effort, sex scenes are gratuitous and awkward distractions except in rare circumstances.