You Know Your Friends are Always There: Comixology
It’s difficult to estimate how large a role Raina Telgemeier’s graphic-novel adaptations of four books in the series (2006-2009) played in this decision. They were well-received critically, and according to a librarian quoted in the aforementioned article, popular with beginner readers.
Telgemeier’s b&w, 9” X 6” gns, at the very least, tested the waters. In a pitch meeting, Telgemeier had described how much she admired the books, and was contracted to do four. Martin herself was initially contracted to do four — although not the same four that Telgemeier adapted. Eventually, Martin and a small team were putting out about one a month.