Because DC made an exclusive e-book deal with Amazon (which is mildly irritating) B and N are pulling their print comics from stores (which is outright juvenile).
As long as retailers keep acting like kindergarten kids (if you are friends with her, you can't be friends with me), buying books is going to continue to be a pretty dismal experience.
What happened to people making stuff and shops selling stuff. All the stuff. In one place, FFS.
(If you still happen to have a comic book store struggling along in your neighborhood, I would suggest getting your graphic novels there....)
Amazon's grip tightens on the entire book-publishing chain