My guess for the main reasons would be:
1) Resources to implement it- every shop I know has dozens of tasks/projects that need doing
2) Time to monitor/administer- deleting SPAM messages, answering questions, etc.
3) Less than critical mass of community members- no one wants to belong to a community that doesn't have anyone in it
4) Too busy focusing on the core business issues- trying to do too much dillutes your efforts and results