>>273100065The serious answer can be traced back to the early 2008's when shows like K-on, Nanoha, Mai Hime, and Lucky Star and franchises like Touhou showed that you can have a very fun and engaging anime without any major male presence.
But what finally set the fireworks blasting was Love Live which had not just zero male presence but also no panty shot. Like, for real. Love Live refuses to show skin and sex appeal except for the most basic and obligational beach episode.
It's success set the stage for the near extinction of panty shots in anime and made things a lot more conservative