Lieberman is more moderate than most Democrats in politics. That's why he had to abandon the party and run as an Independent. There isn't much room in the middle anymore and Lieberman doesn't have much support on either the left or right.
(Politico) — Connecticut Sen. Joe Lieberman — deeply unpopular with voters in his home state — is unlikely to seek reelection, a knowledgeable source tells POLITICO ahead of the senator’s announcement on Wednesday.
The four-term senator’s decision comes after months of speculation on the political future of the independent Democrat.