I think part of the reason for the slower output is the graphic intensive nature of the books. Look at some of the books, even just back to 3rd edition. They are so much easier for publishers and graphic designer to put together. Also, the editing is much more precise now. I find that when going back through the old stuff, especially the adventures, there are quite a few mistakes. (Not there aren't a few in the PHB and DMG, but they are not nearly as glaring.)