It's the same reason why everyone else who have tried to copy World of Warcraft have failed miserably.
Small, multiplayer-only indie games die a quick death, unless they're picked up and pushed by a publisher. Fact of life.
Which brings me to the third sentence. Yes, that is pretty true. But it's usually because they start out as a bit of buggy messes, and people aren't willing to let their pride suffer for it. On top of it, the total initial audience is much smaller.
Bugs + smaller audience = small starting community. Small starting community + normal bleedout rates for non-MMO multiplayer-only games + bleedout rates for games with lots of bugs = quick death. See Gettysburg: Armored Warfare, which after only 3 months on the market is said by the dev to have been entirely ditched by the publisher.
However, I don't see how a 6-8 hour campaign would have alleviated that, either.