because the concept lies entirely in the artist's intent, and a listener without the context of knowing the artist's intent ahead of time is entirely unaware of the album's theme or the story the music tells. I think it's true of jazz - which can certainly convey a mood without context - as well as more subtle works like George Winston's seasonally-inspired solo piano albums.

>http://en.wikipedia.org/wiki/Black_saint_and_the_sinner_lady

interestingly, nothing in that entry describes it as a concept album, just one composition written partially as a ballet. (I have the record and have listened to it a dozen or more times but never contemplated the 'story' the music tells.)