The long and short of it is that there are not sixty votes in the Senate for a public option or Medicare expansion and there never were. The fact that the sixtieth most liberal member of the Senate is now the biggest veto player out there isn't, in my opinion, just or desirable, but that's what the rise of the sixty-vote Senate has wrought. If we had majority rule in the Senate, or if the Senate didn't exist, we'd have a public option right now. But that's not the world we currently live in.
Jonathan Ladd recently tweeted that the two parties seem to do things differently: Democrats are Krehbielians and Republicans are McCoxers. Translated, this means that when Democrats run things, legislation is determined by the median member of the chamber, and when Republicans are in charge, legislation is determined by the median member of the party. Translated further, this means that Republicans know how to enforce party discipline and Democrats don't.
There may be some truth to this, although I think we're really talking about differences in chambers. Pelosi has pretty consistently pushed through as liberal legislation as seems possible in the House. Reid is confounded by the fact that on controversial issues, he doesn't even have a functional majority caucus. There are 59 Democrats. Lieberman sometimes votes with them, unless he doesn't want to. To the extent you need 60 votes to do anything, Reid can't do anything. Applying pressure to Lieberman is useless and quite possibly counterproductive.