Hmmm, nobodies mentioned what is probably the bigget reason.
The common wisdom now is that adventures don't sell. This is borne out by a great deal of sales data, and is pretty much agreed to by everyone in the business. Back in the 2e days, WoTC/TSR was making modules, with the best designers in the game doing work that today goes to the d20 companies.
Nowadays, the best designers are working on sourcebooks, both at WoTC and at the d20 companies. Adventures are secondary, because they aren't profitable enough to have the top designers making them, or spending the time they did in the 2e days (which were terribly inefficient).
That alone is reason enough for the quality shift you see, leaving aside issues of crunch vs. fluff.
This is my theory - though in interest of full disclosure, I almost always make up my own adventures, and am not a big buyer of modules.