It's the nature of the beast.
The Republican Party--at least until now--has tended to be more homogeneous in ideology than the Democrats. They've always been better at marching in formation and saluting when told to by their leaders.
The Democrats are, by necessity, a coalition. Labor unions, immigrants, minorities, wealthy Upper-West-Siders, tree-huggers, gays--they band together loosely and sometimes get along with each other in hopes of persuading others in the party to back their own agendas.
It's a fragile construct, and when confronted by something enormously complex like health care reform, interests collide and it can easily break apart. With exactly sixty votes in the senate, there is no room whatsoever for error.
Down deep, I think President Obama is willing to settle for anything, as long as it's written on a piece of paper he can sign with a flourish, take credit, and move on to other priorities.
Maybe the Republicans are right when they say the Democrats can't govern. Then again, I haven't seen any remarkable statesmanship out of them over the last decade, either.