Recent sales figures state that Batman: Arkham City shipped 4.6 million copies.
To my knowledge, exactly zero of those copies came packaged with download codes for free digital comics. Zero came with minidiscs or USB drives pre-loaded with digital Batman, Robin, Catwoman, Nightwing, and Gotham Central comics.
My copy, and so I assume about 4,599,999 other copies, came with a catalog advertising a wide variety of Batman merchandise, including toys, clothes, and posters. Comics and graphic novels are mentioned on exactly two pages. The last two.
DC/Warner Bros. has enough awareness and marketing savvy to release the new Batman DVD movie on the same day as the eagerly-awaited video game, but for some reason continues to miss these easy opportunities to introduce new people to comics, thereby increasing their chances of selling more comics. This one game reached, what 20 times the audience of the most recent Batman comics? Why didn't it come with comics?
This isn't exactly rocket science, DC.