Justin Alexander asks on his blog Whither the Dungeon?. The subtitle is "The Decline and Fall of D&D Adventures", although I should mention he specifically means "knowledge of how to run adventures". Although indirectly, I suppose the trend he describes affects the quality of adventures as well, as the people writing adventures now are those who never learned how to run adventures, so they don't know what to include or how to present it.
I wonder, though, if there is some additional reason for the decline of adventure writing. But perhaps that is a topic for another time.