1) Most of the best written modules end up in Dungeon. I have no idea why some of the companies throw money behind some of the drab poorly written brainless hack and slash modules that end up getting produced. People who want a good module go buy a Dungeon. People who want to sell modules to often want to sell thier own average to inferior work, or one of thier friends average to inferior work, instead of the work of a handful of guys in the business who really do have the right stuff. It is a publishing business. It is about the authors. Duh.
2) It is simply very difficult to write up a full length modern style module in 32-64 pages. Almost any 'classic' module that came out today would be panned as an amatuerish effort, in part because of this constraint and in part because we demand so much more of a module today. So much flavor has to get cut for space that even a good professional module has a hard time appealing to a DM who is himself probably a pretty decent writer. Modules just don't inspire DM's the way they did in the early '80's and noone had seen anything like this before.
3) The corrolary of #2 is that a well written deep full length professional module has to be expensive, and this turns off alot of the potential customer base fairly or unfairly. "I bought modules for $7 bucks, in the '80's. Why do I have to shell out $25 now?"
4) Every module that is published is competing against every module that was published before. When modules first came out, one module occupied nearly 100% of the supply of a given module of X level. Today, there are 20+ years of modules to draw on.
5) I agree with Mark. There are alot more groups playing at above 10th level than ever, which means any given module is alot less likely to be adaptable to where the group is now.
6) Designing your own adventure is alot more satisfying.
7) One of the things that the setting search proved is something I've always said. The guys that publish modules/settings aren't necessarily the best in the hobby. They are simply the guys that put the most effort into getting themselves published, and/or were in the right place at the right time. One thing the Pro's are going to have to accept, is that for every Pro out thier publishing a superior product, there is another Pro making money whose material is probably even below average for DM's as whole, and there are probably 2-3 DM's not publishing for whatever reason that have just as good ideas. Given that, it shouldn't be too surprising that any given module, which is a relatively little ammount of work, gets bought by its customer base of DM's. Source books target a broader market (DM's and players), represent a larger ammount of work (in terms of pages), and have the advantage to the DM of elimenating the need to write up a large number of house rules and flavor context to share with the players. Even if the DM could do it himself, he is willing to buy someone elses labor for the advantage of handing his players a book and saying, 'Ok, this is the starting point. I'll inform you of any changes.' With a module, that text is only meant for the DM, so a DM can use a much sketchier write up for himself if he knows basically what he wants to do.