One anecdote does not a trend make, but it does one data point make.
16 months ago, I released an adventure in PDF. It was world-neutral. It had tons of interesting NPCs. Re-used bits of OGC from a dozen different sources, and had no small amount of new stuff in it, too.
To date, it has sold a whopping 16 copies. Not this month. Total. Ever. It appears that a vocal minority on internet messageboards clamor for adventures, but when it comes time to actually buy an adventure, there is little to no market for them.
So to be honest, I have very little incentive to work on adventures, because, to be honest, people just don't seem to want to buy them. I suspect it's a combination of (a) there are more players than GMs, so by limiting a product's utility to GMs only, I am restricting my market, (b) most GMs disdain published adventures, preferring to "roll their own" adventures anyway, and (c) as has been elsewhere noted, Dungeon has a pretty solid hold on the adventure market - you can get three or four quality adventures per issue - WITH the WotC material nobody else can use tied in - for a very low price (thanks to the fact that Dungeon can pre-sell their adventures in the form of subscriptions AND has the luxury of extra ad revenue that it's difficult to get in published adventures).
With Dungeon having so many advantages, and the market being so small to begin with, the Return on Investment just isn't there, and I suspect that's why you see so few adventures.
