Through the '80s and '90s, a typical year saw three R-rated films in the year's top ten earners at the American box office, and eight in the top twenty.1
During the 2000s that number dropped sharply, so that the average was more like one of the top ten, and three of the top twenty in any given year were R-rated. Since 2010, R-rated movies have been even rarer than that in the upper ranks.
Much of it would seem to be a reflection of the transformation of the action movie during these decades, from R-rated shoot 'em ups to PG-13 rated CGI spectacles about superheroes. Along with the even more complete disappearance of the sex-themed blockbuster, this translated to a major change in the market. Today's R-rated successes, by and large, are comedies like last year's The Heat (#15), We're The Millers (#16) and Identity Thief (#20), horror films like The Conjuring (#19), and Serious Adult Dramas like American Hustle (#17)--lower-budgeted and lower-grossing fare for the most part.
1. The average was actually 3.1 of the top 10 and 7.8 of the top 20 in the years 1980-1989; and 2.9 of the top 10 and 7.6 of the top 20 in the years 1990-1999. By contrast, the number was 1 of the top 10 and 2.9 of the top 20 in 2000-2009; and 0.5 of the top 10, and 3 of the top 20, in 2010-2013. All calculations based on data from BoxOfficeMojo.