My guess is that there are two main "reasons" that explains it:

- Sanderson didn't work on the whole book but by story line. He had spent a year writing Rand's and Egwene's, was just beginning Perrin's, He knew this part of the story intimately, but had not delved much in the Mat-Perrin-Elayne and the many secondary plot lines. He had not reached the phase where he could look at the book globally and made a bad call. His division worked for TGS, for the most part. It's when he got to the rest of the material he had not worked in depth with yet that the problems started. Sanderson himself would have much preferred finishing everything first, but...

- Harriet ideally wanted a book out that year - they were looking at what Sanderson had finished with that in mind. The fans had been promised the book. Sanderson was all too aware of this, of the disappointement this would cause - and by how long the book would be delayed (2 years, probably), and they were wracking their brains for a solution, and Sanderson came up with one. It had to be decided fast, the deadline was coming. On the surface his solution worked, it's with deeper analysis of the material he could have seen this really didn't work at all.

Jordan would not have made the same mistake. He knew his own material much better, and the structure of AMOL already existed in his mind. Unlike Sanderson and Harriet, he would have seen the problems such a division would cause. For this, for the fact he didn't work chronologically but around the "turning points", and also for the fact this was his last chance to include everything he wished and some opportunities for minor resolutions would show themselves only once the book was very advanced convince me Jordan would have insisted on finishing it, or get very close to that, before considering how to publish it. Of course this would have ended up being published as volumes, and I doubt in Jordan's hands it would have been more than two big ones. Sanderson's and Jordan's pace are very different. Jordan makes a lot more ellipses, and had scenes that dealt with more issues. Sanderson shows more than he alludes, shows on-screen action Jordan would have just refered to in other POVs. He feels more "fast paced" than Jordan because he uses shorter scenes. but he develops the story through many more scenes than Jordan would have used. Though he's not as bad as Kevin J. Anderson, Sanderson also has a bit of the same weakness: he repeats himself a lot, repeats information he's given in the last POV from the same character. Some stuff Jordan would have mentionned once (if in a longer fashion), Sanderson repeated five times (what I mean by this is very obvious in the Perrin chapters and the Aviendha chapters in particular). Jordan's AMOL would have been far less "step-by-step", in the same vein as the recent books: long scenes, but making many more ellipses in the action itself.