I think this is true to a large degree. If you have a regular group then once you've run an adventure that's it; you won't need to return to it unless your group completely changes. Also, only the GM needs to buy the adventure, which instantly reduces the size of the market.I had a friend who used to work for a large gaming company (I won't say which one) and he had said that the reason many companies had scaled back on adventures, especially the smaller scale one-shot module type adventures, is that they just don't make money.
There's more value in books that combine adventures with a setting, or settings that have lots of scenario hooks, and this is what we've tried to do with Dara Happa, Pavis, and Cities of the YK.
But its still a smaller market, which reduces the revenue. Conversely, writing a scenario is a complex affair; writing a setting book is more straightforward.