I'm curious as to WHY it's turned out this way, though... I wish every show could get a cast album but if it comes to a choice between recording a new or previously un-recorded score, and doing a re-recording of a show, I would always opt for the former.
There is no hard and fast rule that every revival gets recorded. In fact for many years, revivals were not recorded. The label that had the OCR simply used the revival as a chance to promote the original album.
Labels generally do re-recordings of shows only if they feel the sales will support them. RCA Victor recorded the Lincoln Centre revivals in the 1960s because it was a chance to get new stereo recordings of classic shows, sometimes featuring original stars (ANNIE GET YOUR GUN, CAROUSEL, KISMET.)
SWEENEY TODD was recorded because Sondheim cast albums sell well so Nonesuch and PS Classics have been recording the revival casts of all his shows.
PAJAMA GAME was recorded because Harry Connick is one of Columbia's artists, and because the OBC was made in 1954 and recording technology has developed a lot since then. (PAJAMA GAME has not been recorded a lot: It has the OBC, the long out-of-print original London cast. the film soundtrack (missing 3 songs but including all the dance music), and the Jay 2 CD studio cast.)
Last season LA CAGE was not recorded because there already was a perfectly fine OBC recoding (digital too - one of the very first DDD cast albums) and despite pleas from fans it was considered a risk to record. SWEET CHARITY did get recorded but when the show closed abruptly sales dropped off.
THREEPENNY OPERA has the 1954 off-Broadway cast (on Decca), the 1976 Raul Julia version (on a Columbia Lp only), The Donmar London production (on Jay) and the 1989 film version (MACK THE KNIFE - on CBS). The 1989 revival with Sting (who was a big name at the time) was not recorded either.
Cast changes in long running shows pose another problem. Though the WICKED airheads would love it, can you imagine how awful it would be if Decca had to re-record that show every time the cast changed?
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