Direct market-only basically means it's only available in comic book stores. I have no idea why Marvel does it myself, but they only do it to a small handful of books.
Still, my point is that it's a book that's the exception, not the rule. To make the generalization from it that you can't find comics online at three dozen different retailers is simply erroneous. Pretty much ever bookstore chain location I visit these days has its own graphic novels section. And you specifically mention both the Big Two, but like I said, unless I'm mistaken, none of DC's trades are direct market-only. Try to name one that isn't listed on Amazon.