Well, the OBC would have been recorded for vinyl and (maybe?) cassettes. They couldn't even get all the music on the original album. I'm not at all surprised that they didn't want to take the time to record the monologue? Which five minutes of music would you delete from the album to include it?
The ASSASSINS comparison doesn't really work because the entire finale of the show hinges on that scene and the entire album would not have the impact that it does without it. Aside from the fact that ASSASSINS came along in the age of the CD, losing the monologue from A CHORUS LINE doesn't really impact one's experience of the album. You still get it. You just don't get much of Paul's story, just as you don't get much of the stories of many of the other characters.