Because the leader of the Democrats is still Mr. Harry Reid, a useless, spineless Democrat who does not actually care about passing liberal legislation, at all. And the most powerful Democrats in the Senate are, of course, the "centrists" who refuse to support any bill until whatever the Republicans want done to it is done to it (that is centrism!). And two Senators are near-death and hence not able to vote for anything.
Here is Harry Reid's press secretary:
"While this is, of course, good news to the people of Minnesota, President Obama, and the Senate Democratic, Franken's mere presence alone will not mean that the Democrats will be able to jam through our agenda, or make it any less critical for Democrats and Republicans to work together."
Well, it is not actually critical, anymore, to get Republicans to "work together" with Democrats. It is actually only critical for Majority Leader Harry Reid to get Democrats to actually vote with Democrats.
But Bernie Sanders has a crazy idea: a straight up-and-down majority rules vote on health care legislation! Achieving this, of course, requires a 60-vote supermajority for cloture, which would be a de facto vote for the health care legislation. Also that would be socialism.