I think a big part of D&D's success is the support, which includes adventures. Having adventures around means it's fairly easy to get going playing the game, and they also provide examples of how to do things. As a counter-example, I've been eyeing Trinity Continuum: Aeon lately, but the breadth of the setting and the lack of adventure support has kept me from taking the leap.
And I think a big part of what makes adventure publishing possible, or at least relatively easy, is the class/level system. If you look at a more skill-based system like the aforementioned Aeon, it's really hard to determine what sort of difficulty is appropriate. You could use XP as a gauge, but XP can be spent in so many different ways that the power level of two different 30 XP characters can vary wildly. But in D&D, you can be pretty sure what a 5th level party is capable of, so it's... well, maybe not easy, but doable to write an adventure for a random party of 5th level characters.
D&D has more pre-made adventures for DMs to use/crib/be inspired by than any other game (at least that I'm aware of) by many orders of magnitude.
Most games have an intro adventure in the book and that's it the GM is expected to come up with the rest. Some have a book or two of adventures - and again that's about it.
D&D has multiple adventures at almost all levels (sure the lower and mid levels are much better represented but even higher level ones are not uncommon).
This was certainly the case from early TSR era and has continued to today.
This wide availability of adventures combined with/thanks to the level system is one of the "secret sauces" to decades of success. And yeah it's from early TSR, but it has worked - why would WoTC change that!