I think because Twitter's energy is different. People intend their forum accounts to be relatively anonymous, but expect twitter to be a place where you show your name and face. Twitter is more personal.
Yeah, and while some people talk big with no credentials/experience to back up the talk, there are a lot of people I don't know personally that I follow whose opinions I trust more than some anonymous forum commentator.
That being said, I deleted Twitter in January and am much happier for it.
