“The original impetus for the open licensing of the d20 System involved the economics of producing role-playing games (RPGs). Game supplements suffered far more diminished sales over time than the core books required to play the game.” Wikipedia, “d20 System“
I think I sorta pissed someone off years ago by making this exact point. My opinion was based on the experience of watching GDW struggle to fund RPG supplements during my time there, and then TSR suffer the same.
The catch-22 is that (a) without supplements, customers lose interest in an RPG, and distributors don’t help by telling retailers your game is dead, but (b) only GM’s buy most supplements, whereas most everybody in the game group buys the game itself, so you’re doing the same effort (with, for example, the same cover painting expense), for a fraction of the market.
By handing off supplement publishing to an ever-refreshing pool of small companies (many just gamers with a one-book dream), WotC dodged that expense and enjoyed the unencumbered sales of core product.
Things like the “Community Content” section of DriveThruRPG are a more recent example of that farmed-out support.
I ain’t sayin’ this is a bad thing. It allows for a lot of diverse creativity. But it also explains why every new edition of a core game sets off tectonic waves among the small publishers putting out sourcebooks and adventures.