When you look at the first half of the 20th century, movies like Gold Diggers of 1933, Footlight Parade, and the Road to ... movies were incredibly popular. You'd see elaborate choreography in Busby Berkeley musicals and Esther Williams movies. However in recent years, maybe one major studio releases a musical per year, and the choreography isn't nearly as elaborate as in movies made 50 or 60 years ago. What is responsible for the decline in popularity of musical films over the years?
I wouldn't say their popularity has declined so much as movies have varied the types of story they tell and genre over the years. pitch perfect grossed over $60 million nationally, jersey boys grossed $40 million. these movies are still doing well, they just have become more focused studio efforts.
60 years ago, musical movies were part of the popular style because in many ways movies were attempting to imitate the theatre. since then, realism has proliferated in our entertainment.