I believe it is designed to allow rehearsals, changes, press interviews and such to be done the weekdays.
It would seem most shows fine it better for business once they start running to play Sunday afternoons and take Mondays off. I wonder if any studies have been done to compare ticket sales for the two performance schedules.
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