Brigid Alverson has a thorough post mortem that can’t really be improved upon, so I’ll just send you there:
There are a lot of reasons why Diamond Digital didn’t work, but I think chief among them is the initial concept was flawed. The idea wasn’t to provide readers with a simple, easy-to-use digital comics service; it was to protect brick-and-mortar retailers by providing them with a digital comics service that wouldn’t compete with them. That drive to avoid competition resulted in a clunky and almost-unusable platform. Meanwhile, comiXology took a different tack and expanded the comics market, bringing in new readers — who then found their way to comics shops and bought print comics.
So yeah, the protectionist era has been laid to rest. I think the idea itself was well intentioned, but in practice it never took hold.
It’s also a moment to reflect on the digital storefronts that have come and gone—graphic.ly and Panelfly—or never even come—remember Longbox? Iverse and Dark Horse are still in there kicking, and Sequential is a more recent and selective app. (Of course there are also iBooks and Kindle.) While you must give Comixology major props for staying the course that they believed in all along, now that this channel is so well established, I wouldn’t be surprised to see other specialized vendors set up. But to be successful you need to believe in what you’re selling, not fear it.