Jan van Leyden
Adventurer
The reason for not publishing printed sequels (or APs) is that you're then really only selling to a subset of the people who bought the previous volume in the series, which means an ever-diminishing number of sales. Given that adventures don't seem to sell too well in the first place*, you don't really want to be getting into that.
But this doesn't have to be the case. Given that many GMs like to tinker with the stuff anyway and given the infrastructure established for 4e (Adventure Tools) it should suffice to define the interfaces of the parts.
Give me information on necessary location (need a swamp nearby?), knowledge (PCs need to seek Aaron the All-knowing, who may be allied with Ernie the Evil), hardware (map to the Tower of Tim the Tomcat) to get the adventure started.
Give me a clear list of goals for the adventure, of items to be won and friends and enemies to be made.
Give me this information openly before I decide to buy the thing. With this help I can evlaluate how well a module fits in my campaign and what changes I'd have to make.
One could orchestrate the preparation of the adventure without investing lots of work, and the players won't notice that they are about to play Adventure 13 from the Saga od the Squealing Swine AP.
I'm in a similar situation right now, changing Madness at Gardmore Abbey for my group playing in Ptolus. I bought MaGA because of the praise it has received, but without knowing what I had to do to fit it into my campaign and whether it would be possible at all.
I'd rather see a lot of adventures on the shelves which I can assess for my purposes than just one I'd buy at my own riks.