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Bands and/or artists making albums.
Before the invention of the CD, albums often told a story from one side to the next with the songs. Then the CD came along, and this changed somewhat. Now, with MP3s taking over, an actual studio album like this is virtually non-existent. Artists now focus on singles alone and never the collection of songs as a whole.
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1) Concept albums were never very popular in the first place. They were fairly rare outside of the progressive genre. Because the progressive genre is not a very popular today, that's one of the reasons not many concept albums are released.
2) Just last year, a pretty popular group released a concept album that sold over a million copies:
http://en.wikipedia.org/wiki/The_Black_Parade
And quite a few more obscure bands have released concept albums in the past 10 years, such as Flower Kings, Dream Theater, Opeth, Spock's Beard, and Mastodon.