Michael Kozlowski writing at Good Reader says that Marvel is retreating from retail distribution to focus on digital:
Marvel is endeavoring to focus almost exclusively on digital distribution to maximize revenue. In the past few months the company has pulled their comics from bookstores all over the world. You would be hard-pressed to find anything but the odd graphic novel in your favorite bookstore, such as Barnes and Noble.
Marvel is finding that they are selling more comics online, than they are in the retail environment. Comixology is their main partner in the digital sphere and they have dedicated reading apps on every major platform. They have sold over 125,000,000 comics since 2009, most of them from Marvel.