Sequels/Remakes.
This is also misleading statistic. All it says is that the highest grossing films have been sequels/remakes, not that studios haven't always cared about sequels. The fact that most of the movies in the top 1981/1991 had sequels. Hollywood has always loved sequels, this isn't some new phenomenon, they've been churning out sequels for a century now.
Also, if Father of the Bride is a "sequel" so is LOTR: Fellowship of the Ring. (Also can argue that Beauty and the Beast, Silence of the Lambs, Robin Hood, and Addams family, are sequels, by that lists logic.)