Last week I decided that I would leave my contract position with Wizards of the Coast. I am no longer working on Dungeons & Dragons, although I may provide occasional consultation in the future. My decision is one based on differences of opinion with the company.
Check out the his full post here: http://www.montecook.com/cgi-bin/page.cgi?montejournal
Also, the RPG Examiner has collated some responses from WOTC itself: http://www.examiner.com/article/why-did-monte-cook-leave-the-dungeons-dragons-5th-edition-design-team