I'm sure Martin doesn't write the books strictly chronological, so cutting it in half would be a problem with timelines. And we had something like that with Feast and Dance, leading to books which are missing whole character arcs.
Separation of books is done in some countries by the publisher, but only after the book is finished and released. For instance in Germany every book is separated in two parts, leading to ten books now, instead of five. Not sure if that's the reason, but with each of the ten books sold at a normal prize they can double their income.