The music industry keeps getting bigger, but the hits can't keep up.

Adele's latest album, "30," was the best-selling record of 2021, and its success showcases her enduring popularity.

The British singer finished December at the top of Bloomberg's Pop Star Power Rankings and was one of the ten biggest artists on YouTube and Spotify.

When Adele released "21" in 2011, at least ten albums sold more than 1 million copies. The list included Michael Buble, Lady Gaga, and Lil Wayne.

But "30" is also a modest hit by Adele's standards. It's on pace to be her worst-selling album since her debut.

The sales for "30" reflect a collapse of album sales industrywide over the last decade. Adele was the only artist to exceed 1 million copies last year. "30," her fourth studio album, sold twice as many copies as any other despite its mid-November debut.

Over the past few years, the number of artists who sell albums in great volume has narrowed to Adele and Taylor Swift. They are the only musicians to sell more than 1 million albums in the U.S. since 2018.

The obvious culprit is streaming. Sales refer to individual purchases of an album (CD, vinyl, or digital), data provider MRC also ranks the year's biggest albums based on overall consumption. That includes album sales, song sales, on-demand audio streams, and video streams. As more people stream, traditional sales have declined.

This collapse in traditional sales means the biggest artists aren't making as much money from their recorded music. While music fans are streaming the equivalent of millions of albums, online listening doesn't pay musicians as much as a vinyl record, CD, or digital purchase from Apple Music.

This also forces musicians to release music more often to stay relevant.