From Brandon Sanderson's website:
Question: Why a delayed ebook release for A Memory of Light?
Answer: This is not my decision or Tor's decision, but Harriet's. She is uncomfortable with ebooks. Specifically, she worries about ebooks cutting into the hardcover sales. It isn't about money for her, as the monetary difference between the two is negligible here. It is about a worry that her husband's legacy will be undermined if sales are split between ebooks and hardcovers, preventing the last book of the Wheel of Time from hitting number one on either list. (Many of the bestseller lists are still handling ebooks in somewhat awkward ways.)
As the last books have all hit number one, she doesn't want to risk one of these not hitting number one, and therefore ending the series on a down note. (Even though each Wheel of Time book has sold more than its predecessor, including the ones I have worked on.) I personally feel her worries are unfounded, and have explained that to her, but it is not my choice and I respect her reasoning for the decision. She is just trying to safeguard Robert Jordan's legacy, and feels this is a very important way she needs to do so. After talking about the issue, we were able to move the ebook up from the originally planned one-year delay to instead come out this spring.