I tried to ask Card myself why no ebook version of Shadows in Flight by contacting him through his Hatrack.com site. An assistant responded: “Actually, Mr. Card was involved in the decision to delay the ebook version until 2013. It will come out in January along with the paperback so it won’t have to compete with the hardcover.”
That’s one way to go. But is there really a net benefit to gratuitously annoying fans when the marginal cost of “printing” each new ebook copy, once it has been formatted, is zero?