What do you expect?
This outgoing Congress isn't about Obama; nor is it about a "mandate for reconciliation" or a "change" from the Bush years. This is the same Congress that refused to impeach Bush and Cheney even when there were obvious charges (much beyond lying about a blowjob) against both of them (and potentially other members of the cabinet) which to his defense Kucinich brought forward and which this Congress rejected.
This is the same Congress that capitulated on war funding because they were afraid of having spines.
This is the same Congress that allowed a downright Nixonian FISA amendment to become law so that they could take their (kindergartner?) recess.
This is the same Congress that allowed wrote a blank check for the abominations of the Bush administration to continue virtually unabated from torture to bailouts to mistreating veterans, and you are surprised that they kept Lieberman?
This is the same Congress that was afraid to even try to pass anything too controversial because of procedural trickery called the filibuster.
But they passed a minimum wage law! They tried to pass immigration reform, and they sort of passed energy legislation!
They didn't do anything worth writing home about. The Congress of 2006 to 2008 may be known in history textbooks as the "capitulating Congress," or maybe just more appropriately "Cheney's bitches."
Not only that, but their first act upon gaining an even larger majority was to spit in the face of the loyal Democratic base. If they had stripped Lieberman of his chairmanship, and at worst removed him from the caucus, they would have angered Lieberman but not many actual Democrats; instead by maintaining the status quo, they offended loyalists and fractured the base (albeit probably only temporarily).
Even I thought Lieberman should be kicked out of the Democratic caucus or at the very least stripped of his chairmanship... and I'M AN INDEPENDENT!
You can say what you want about how Obama weighed in on this decision, but Obama wasn't the one ultimately voting on Lieberman's fate, and Obama had much more to gain in the long-term from reaching across the aisle than the outgoing Congress.
I have utmost confidence in Obama and his coalescing administration... the Congressional leadership--on the other hand--not so much. From where I'm standing, it looks like Obama's administration is going to have a lot of babysitting to do on the Hill, because a lot of these people who voted to keep Lieberman and who patently refused to hold the Bush administration accountable are coming back next year. These people are the reason why I don't identify as a Democrat... but ironically I find more offense than they with the fact that they retained someone in their caucus who openly questioned their patriotism, which--judging by their recent super-secret super-serious vote--is a form of Liebermasochism I can only assume that they endorse.
Oh well, Congress figures that like their goofs over the past few years (decades?) we're just going to forget about it and move on. Which is probably true.