Because demonstration is an excellent way to teach others how to do something. A new setting, where the book itself walks through its creation, would empower DMs by showing them even as it tells them.
But personally, I don't think new is strictly necessary. You can do this with old settings too, if you're reinventing them. The only meaningful difference I can see is that, with a new setting, everyone is new to it, so they kinda have to read about it to know what it is and contains.
I find it difficult to relate to or care about a map alone. For words to have meaning and names to have power, we must be brought into their home, their world, their time. A map alone does not do that. It leaves me just sort of staring. I can, eventually, get something built up that makes use of that map. But it would be easier, faster, and more productive if I had more to work with, and the map is not that useful once I have that "more."
All I can say is, things like the Dawn War, the tragedy of Arkhosia and Bael Turath, and the loss of Cendriane stick with me far better than any map could. A map has its uses. It doesn't tell me why I should care about any of these places, nor what is(/was) happening in any of them.