I think the reason is that internet is full of trolls and if you don't put significant effort into making community / collaboration clean and vibrant then the collaboration site is destined to fail.

For example, compare Quora and Yahoo! Answers. I don't thing Quora is successful because they have very good UI (it does not hurt), I think they are successful becasue they are very serious about making they site clean and without trolls. On other hand, Yahoo! Answers is 'troll heaven'.