What are the most common reasons?
It's say the most common reason is that they didn't start with building relationships. People try to shortcut it and just build a community by mass importing large groups of people into a conversation platform and expect the relationships to just happen.
People only participate in communities where they feel like by becoming a trusted member of that group, they increase their own value in some way. That only happens if they care about the community. They're only going to care about the community if they feel like they're a part of something, and have built relationships there.