As odd as it may surprise you, it's not about someone actually neglecting the importance of a community. It's also not about lack of content. Or lack of will. Or lack of professionalism.
Many community managers know communities are important. They have lots of content. They're willing and able. And they're über professionals. But...
... Sometimes they're just trying too hard. Trying too hard to sound happy, or sad, or nostalgic, or enthusiastic, trying too hard to show they care about cooking and cars and art and sports.
So many communities fail because community managers talk and act like someone they're not. It's simply not natural. And people are getting increasingly better at spotting bullshit.