https://www.engadget.com/2017/05/12/mp3-is-dead-long-live-aac/

MP3, the format that revolutionized the way we consume (and steal) music since the 90s, has been officially retired -- in a manner of speaking. The German research institution that created the format, The Fraunhofer Institute for Integrated Circuits, announced that it had terminated licensing for certain MP3-related patents...in other words, they didn't want to keep it on life support, because there are better ways to store music in the year 2017. Rest now forever, MP3.

In its place, the director of the Fraunhofer Institute told NPR, the Advanced Audio Coding (AAC) format has become the "de facto standard for music download and videos on mobile phones." It's simply more efficient and has greater functionality, as streaming TV and radio broadcasting use the format to deliver higher-quality audio at lower bitrates than MP3.