The first scenario I had published was one of the chapters of 'Rogue Mistress' for Stormbringer. There were about 7 or 8 writers working on that project, and it took about 9 months to get the first draft together, from initial notes and briefing, through to first draft manuscripts. Then the whole thing got dropped, because it wasn't working as envisaged.
Then a new editor, Keith Herber, came on board, picked it up, contacted all the authors again, suggested a shed-load of changes, and we started work again. Took another 6 months. It finally came out a good year and a half after it was first conceived.
For a big campaign pack, akin to Pavis, Rubble, Griffin Mt, Borderlands, etc - that's not untypical. And working with multiple authors can be a nightmare; if one misses a deadline, the entire project has to wait. If an author drops out, someone else has to fill-in. Or the project gets rejigged to compensate. If its a single-author, the sheer volume of work required is extremely daunting. I'm no newcomer to penning big, involved campaigns, but it still daunts me when I consider the scale of work to be done (and the Dara Happa campaign is probably going to be such a project. Gulp).
There's then the task of discovering if it works. Rules and settings are relatively quick and easy to verify. But a scenario... is the pace right? Does the plot hold together? Are the NPCs correct? Where are the plot breakpoints? Is it fun? Playtesting takes a lot longer, and you can bet a decent playtest will bring up all sorts of issues that may necessitate a lot of rewriting to fix, owing to the interdependency of the material.
The upshot is, settings and rules are quicker to turn around and yes, they sell. I share the feeling that games live and breathe with their scenarios, but putting out quality scenarios (especially the big buggers) is a tough old job, and obviously, as a writer, you want to get it right.