Because that would have been even more helpful to me, and I'm sure there are people who may have had that intention and then got overwhelmed by how '14 started with building an entire world, or didn't have the benefit of having been a player before and thus having even less of an existing knowledge base.
If I were coming in completely fresh faced, I would definitely assume a thing called the Dungeon Master's Guide would be the resource to start that learning process with. Why wouldn't the guide to being a DM teach me how to be a DM? Even if historically this book has not been that thing, if that is a problem that is consistently happening (and it certainly sounds like it is, both from readers and the writers) for understandable reasons, then addressing that issue is a worthy consideration in this update.