Because he is a Democrat, and he is being true to his party, even if the current crop of dummies forced him out to make an anti-war statement. And when he runs for re-election again in six years, I'd bet it will be as a Democrat again.
Run. From now on Lieberman can coast through any election.
He's untouchable.
Why even mount a campaign against him. If you beat him in the primary. So what. He'll run and as he has proven, still win.