Thinking of the Hornby Catalogues of the 80s, I remember there was a coach and a brake for the Big Four, the LNER were based on Thompson but the other three were Mk1 based. There was also I believe MK1 and Mk2 for BR. These were available every year and I suppose allowed the right colour coach to be pulled by your locomotive. Lima at the time did MK1s of four types including Green, Chocolate/Cream and Maroon but with BR markings.
Today we expect more authentic stock and I suppose these must be produced in fixed numbers so that when/if they sell out that is it until a further batch is ordered. If other designs are being produced, the factory may not have the capacity to make that batch.
The fact that production is now based in China and that unsold stock in a warehouse doesn't fit in with today's business models. will mean that it may longer not be possible to always buy the new stock that might be required for whatever locomotive.