And now I will pound my own head into pulp in frustration:
So why did it [voting on extending Tax Cuts, except for Rich People] fail? According to a very plugged in Senate aide, Senators debating the issue were very aware that the polling was on their side. Yet, paradoxically, this ended up tipping the balance against holding the vote. Senate Dems felt they were alreadly winning on the issue, and in the end they thought a vote risked upsetting a dynamic that was already playing in their favor.