It was suggested, on a thread when the original Bachmann model was announced, that maybe the 8-car Western Region set would have been just that little bit too long for most prospective buyers, that is, their layouts may not accommodate them. It would also have been even more expensive than the 6-car Midland set that was produced.
It would have been risky to have sold cars individually and not as complete sets - some might sell better than others, leaving unsold and unsaleable stock. It's a complete unit train, not something that was designed to be remarshalled. (Yes, that happened to the real thing in later life, I know.)