The earliest attempts at out of game mediums for Blizzard consisted mostly of backstory and sidestory. The War of the Ancients Trilogy talked about history of the Night Elf heroes. Tides of Darkness just rehashed the cannon plot of Warcraft II, likewise Through the Dark Portal did the same for the expansion. The Sunwell Trilogy just gave the specific minutia of the Magister's Terrace and Sunwell instances. Arthas: Rise of the Lich King rehashed a lot of plot lines from WCIII, and gave some insight into Jaina and Arthas, but it wasn't anything that wasn't also apparent in the questing in Icecrown or the ICC 5 mans. The Death Knight Manga told us the specifics of the backstory that was covered in quests in both the DK starting zone, and in Northrend.
This is an example of what out of game materials should be: supplemental, optional, an enhancement to the plot already in the game. They should cover stories that are already portrayed in game, or stories that don't have enough significance in the major narrative plots in the game to be done justice in game. The Death Knight Manga was a particularly good example in my opinion. Playing through the game, I knew that Thassarian was a cool character. I knew he became a DK when he shipped over to Northrend with Arthas' expeditionary forces. I knew he had a skeleton buddy named Lurid. I knew Falric and Marwyn were captains under Arthas' expedition. This is enough to know in game to propel the plot. It's still nice to know the little details, but that's what the Manga was for. It was in the Manga that you find out that Falric went searching for Arthas after they defeated Mal'ganis, and when Falric failed to return with the prince, Thassarian went looking for both missing men. He found Falric in a cave, already turned to the Scourge. Falric turned on his former charge, and slew him, raising him as a Death Knight. It's a good story about the fall of a good man, but Thassarian isn't as important to the plot to warrant having such small details handled in game, at the expense of moving other, more important story arcs along.
Then came the WoW comic. A comic is a different animal than a book or omnibus manga. In order to produce a successful comic, it has to be something that people are willing to purchase every week. In order to get a fresh weekly comic off the ground, you need a killer story. So Blizzard took one of the biggest hanging threads from in game, and sold it out to the comic, The Missing Diplomat. The Missing Diplomat was an Alliance side quest chain that led the character in search of a diplomat who was kidnapped en route to a peace conference with the Horde. It served a pivotal function within the game, it was the primary means by which non night elf Alliance characters made the transition from Eastern Kingdoms to Kalimdor. It was also known for its abrupt ending. There were little things here and there that led people to believe that it would be picked up in the end game, most notably that the Missing Diplomat, who was actually the King of Stormwind, was rendered in game in the basement of a building on Alcaz Island. All of the opportunities to revisit that quest line were cast aside, and if players wanted to know what happened, they had to go buy the comic.
This began a new era of Blizzard expanded universe media. This is a problem because story arcs that are begun in the game should be wrapped up in game. Jumping the story across multiple formats creates a jarring narrative that will be incomplete for the majority of the people who experience it. Blizzard then began doing this for every incomplete story line that they had in the game. Malfurion's been asleep since WCIII, and Fandral's curiously asking for more and more Morrowgrain while the Emerald Dream corrupts itself and Teldrasil is seeded with that corruption. Green Dragons are rampaging through the world, and Ysera is trapped in the Emerald Dream. That was the status quo through Wrath, and suddenly in Cataclysm, all those threads just disappeared. What happened? Stormrage happened. Cataclysm gets released, and major faction leaders have suddenly shifted, or died, or turned into a giant diamond. What the hell happened? The Shattering happened. The Horde and Alliance are suddenly back at war in Kalimdor? What happened? Wolfheart happened.
Throughout Cataclysm, Blizzard has been outsourcing their lore to books and comics. There are a couple possible explanations for this. They might simply feel like their story is too big to be constrained within the confines of an MMO, which has specific leashes on what you can try and accomplish. They might have made their story too ambitious to implement in game on their budget. A raid into the Emerald Nightmare to rescue Malfurion would have been amazing. Having the fight between Garrosh and Cairne play out like the Duel between Thrall and Garrosh before Wrath would have given some weight to the extremely weak pre Cataclysm event. Perhaps the most insidious possibility is that Blizzard looked at the Arthas, and the success that that novel had, and saw the opportunity to made money at a higher profit margin. It costs much less to produce and publish a novel than it does to do the same for an MMO the size of WoW.
Ultimately, however, by outsourcing the story to these other mediums, it alienates a large portion of the playerbase who do not purchase them for whatever reason. Be it a lack of financial security (cue players calling out "This game discriminates against the poor!" in Commander Shepard's voice), a lack of interest in the other mediums (Books just aren't for everyone), or they're simply busy spending their reading time reading better books (I'd rather read American Gods for the 11th time than read Wolfheart again.).