Maybe that’s because a) older albums havr had more time to reach 25 million sales, and b) CD sales have declined since about the 1990’s.
I disagree.
Without looking it up, I assume most of these albums hit 20-25 million long before now, and long before 2001.
This list includes more than just CD sales, some of these albums were released before CDs existed.
Napster/Spotify/downloads/etc.. is one reason why music sales have dipped the last 20 years.
That, and I think the market is fractured, along with no artist or band doing anything musically interesting anymore to sweep 20-30 million people off their feet.
The music business can't even get the Ariana Grande's of the day to outsell Tracy Chapman, Lionell Richie, or Eminem.
I think Adele is the only current artist on that list, mainly because:
In October 2015, numerous journalists speculated that other musicians had pushed back their album releases in order to avoid chart competition with Adele; artists such as Justin Bieber, Rihanna, Sam Smith, and One Direction all did this so that their sales would not be affected.
and
Adam Shewin of The Independent stated that "25 sent casual purchasers back to the remaining physical stores and may even have introduced a new generation to the delights of ownership."[200] The album was described as saving the music industry, which was seeing dwindling profits with downloading and streaming as the popular methods of music consumption.[111] The album was also noted for boosting sales of retailer Target, which sold the extra track edition of 25, during Thanksgiving and Black Friday. Target Chief Executive Brian Cornell stated that 25 was "the biggest release we've ever had – this is going to break all the records for us" and that "the combination of Adele dropping when it did, the weekend before Thanksgiving, really helped bring in people".
from wikipedia
I'm not an Adele fan, the music is too slow, sombre, and depressing, but maybe someone with more knowledge on the subject can explain why her "25" album was so anticipated? Why did other artists go out of their way to help her with her new album at the time??