You could say the reason is jurisdictional. Everything inside the book is my territory, but for the cover I have shared custody with the marketing department.
Everything inside the book is as authentically Japanese as I know how to make it, and that includes addressing Professor Yamada by his last name. Japanese culture enforces social stratifications on many levels, and so despite the fact that Yamada and Mariko become close friends, there are at least three reasons why she’d never call him by his first name: he’s much older than her, he’s a college professor (or professor emeritus, anyway), and he’s her martial arts instructor.
Once we get to the cover, the goal isn’t to reflect Japanese culture accurately, or even to accurately reflect every detail of the novel. The function of the cover is to sell the book. The consensus was that people reading the back cover would be thrown off by reading “Professor Yamada Yasuo” on one line and “Professor Yamada” on the next, so we followed the Western naming convention and put his last name last.
Incidentally, this is also why Mariko has ten fingers on the covers of Year of the Demon and Disciple of the Wind: people who don’t know the books might be thrown off by an anatomically correct Mariko. (If that explanation doesn’t suit you, scroll back a few posts and look at the happy face on Sayuri Oyamada, the cover model. Can you look her in the eye and ask her to sacrifice body parts for her art? I can’t.)
Thanks for the great question, Marc! Everyone else, I would love to make answering fan mail a regular feature of this site. Ask ‘em if you got ‘em: steve (at) philosofiction (dot) com.