Whenever there’s a significant media think piece about movie scores, I see it over and over again on social media feeds (because of who I follow).
The latest one by Slashfilm—asking where the melodies have gone in film scores—summarizes the reasons pretty well. Enjoy...?
Having not read the piece yet, I'm speculating it's because movies are so much faster-paced now, with no pauses in the action for a thematic piece to stretch its wings. That's my two cents, anyway.