Also, another detail, and that is on quality of the products.
The Forgotten Realms campaign setting is one that I own, the old AD&D one. That book bangs. It is super useful, with a lot of plots and adventure hooks and actual hexes with naughty word in it to do. But, let's look at Dark Sun. Dark Sun is very, very hard to use. The monster books are 10/10 imo, but the setting itself gives you nothing but broad strokes. FR campaign gives you both broad strokes (easy to fill in if you've read lord of the rings) and microstrokes (actual immedietly gamable information like fully developed hexes).
Meanwhile, Dark Sun, Planescape, both of whose box sets I own, just don't give you enough material to use their very alien settings. It takes a lot more effort to engage with these settings and to produce a game than it does the Forgotten Realms. This, to me, makes the FR feel like a higher quality gaming product, and it allows FR as a setting to grow a lot faster as well (especially coupled with the endless amount of novels being pumped out for it).
If Dark Sun and the other niche settings had tried to solve this very difficult-to-solve puzzle of making hyper gameable content to make their niche products more functional, the lifetime sells may look a pretty picture different. This is all hypothetical, and I could be totally wrong, but I do consider this problem of "Usable setting" vs "Alien setting" a part of the narrative in the data.
One last note...I tried to run Planescape two years ago. Extremely difficult to do. I was basically inventing everything, from cultures to environs to settings, because those books really only give you barebones materials.
