The reasoning is, of course, a financial one. They want to maximize their consumer base. Why produce content that portions of your target audience will not find useful? Declared the allegiance to a particular setting? You can still buy our book!
|Tits! You like tits right? Buy our book!|
|Is this the setting where people ride dragons‽|
Sure, it's easily shrugged off from an at-the-table perspective, at-least in the short run, but there are long term affects. Eberron has a metaplot centering entirely around dragons. So, how do the new natural-disaster themed dragons effect that? There's a new Eberron video game, which will introduce the setting to a whole new audience? Will they see the setting the same way people introduced to it back in 3.5 did? Can the Forgotten Realms of today really be considered the same as the one I so fondly remember from Baldur's Gate? If not, that's going to be very jarring if they ever make a third one.
|Oh, but a Spelljammer crossover is fine. Hypocrite.|
I don't mean this from a purely nostalgic point of view either. As a consumer, this sucks. Why buy a setting book, or choose anything other than the default Points of Light setting, if they're all exactly the same save for the relative placement of geography and peoples? It cheapens the brands to the point of irrelevance.
The thing is, it's totally unnecessary. Just because it's in the book or mini box doesn't mean you have to use it. There are ~100 other monsters in the book you could use. Who ever gets to them all? This was the original intent of these supplements, but somewhere along the line the idea of a supplement as something... supplemental became lost. The books ceased to be resources alone which, again, was a business decision. Decisions based upon financial necessity are understandable, but this may have been a short sighted one that may not actually be making Wizards money, in the long run.
|Yes, ninjas! More! Add more stuff, I don't care what! This won't cause our product to become completely overblown at all!|
So, how does this apply to game design as a whole? As the saying goes, "that which does not add, subtracts." Every generic element added to your game makes it that much more like every other game on the planet. While in some contexts this may be your goal, it generally makes your title less noticeable among the masses. In crowded genres, this can be a death knell.