There is lots of unreleased music in the world and as time progresses a lot of it being issued by us and many others. The reasons that it was left in the vault vary, it could have been because too much great material was recorded at the same time and only some of it could come out, it could be that the artists last record flopped and the record company didn't want to put anymore records out by an unsuccessful artist, or it could be that the group were experimenting in the studio and the results would not have sounded great on AM or even FM radio, so what was the point of releasing it. What is true, however, is that it is very rare to get a whole albums worth of unreleased material by a front line act, and when you do its pretty certain that reasons one and three above come into play.