Because the people were fed up that the prior direction they went before the mob action was not standing up for their needs. He talked on behalf of the people, promise to work on behalf of the people, but had no spine to do so.
He compromised for the people on the side of the corporations and insurance companies.
The mob that went in Obama's tide before shifted, because Obama could not lead or he perhaps thought that the other side was actually willing to work with him.
The other side does not care about bi-partisanship and will do anything they have to do to regain power. And POWER is all they really care about.
They will use buzzwords like socialism or refer to themselves as co-heirs of history or any other cockamamie lie they can come up with to get people on their side.
"Don't Tread on ME" really does mean - do not tread on corporate America's right to take our jobs overseas, abuse workers over there, and come back with record profits.