The primary change has been a switch in demographics. While there are still the occasional R-rated movies in the theater, they are few and far between. Before the Internet, Netflix and On Demand, people of all ages would visit theaters for dates and nights out. When cable expanded and there were more fun options at home such as console video games and DVD quality video, the demographics for movies began to shift.
People over 18 were staying home and the the teens and pre-teens, still in that rebellious phase, crept from the home and into theaters. Movie studios realized that by creating R-rated movies they were excluding a large portion of their audience. R-rated movies dropped in revenue and PG rose sharply.
Movies that would have normally been R-rated such as action and horror movies where “dumbed down” so the teens could watch it without having to get adult permission. What that translates to is a boatload of movies that anyone with normal hormonal levels don't want to see. We want to be scared and excited, but most of the stuff in theaters only illicit snores and walk outs.