The way it was explained to me by one label exec, in addition to recording costs, maufacturing costs, sales and distribution costs, each quarter the labels must pay royalties on every CD in their active catalogue - even if that CD has not sold one single copy in that quarter. This explains why some slow selling titles are deleted so quickly.
In some cases the recording is made but it is cheaper NOT to release it than to go to the expense of mixing, mastering, pressing and releaseing a CD that might sell only a few thousand copies.
In some cases there were objections: Noel Coward apparently did not like the cast on Columbia's BITTER SWEET recording and asked the label not to release it. They could have released it anyhow but since he was one of their recording artists they bowed to his wishes.
Cast albums are NOT "soundtracks."
Live theatre does not use a "soundtrack." If it did, it wouldn't be live theatre!
I host a weekly one-hour radio program featuring cast album selections as well as songs by cabaret, jazz and theatre artists. The program, FRONT ROW CENTRE is heard Sundays 9 to 10 am and also Saturdays from 8 to 9 am (eastern times) on www.proudfm.com