Because they're really not happy with how Senate Democrats dealt with their true public enemy number one - Joe Lieberman.
Top names in the left-o-sphere such as Jane Hamsher, John Aravosis and Markos Moulitsas have spoken. Hamsher appears to be driving the discussion.
They contributed to a Democratic victory on election day. They weren't as influential on the decision to let Senator Leiberman keep his committee chairmanship.
We'll see if this is a tempest in a teapot or a major revolt.