Well, part of the issue is that it's rather difficult to write some other sorts of stories into long modules. In the standard meat grinder, what's going on when is mostly dependant on where you are on the map of a physical location, and that's easy to structure. You get a solid "railroading" effect fro the physical layout of the place.
I mean, imagine a mega-module that's based upon political plots - so much depends on the details of the PC interaction with the NPCs, that you simply cannot predict where things will go in the long term. You'll strain credibility if you try to railroad the PCs in such an adventure, so most of the published matierial will probably be moot very quickly.
I mean, imagine a mega-module that's based upon political plots - so much depends on the details of the PC interaction with the NPCs, that you simply cannot predict where things will go in the long term. You'll strain credibility if you try to railroad the PCs in such an adventure, so most of the published matierial will probably be moot very quickly.