Forty-five percent of the Democratic base now says they aren't going to vote in 2010 or are thinking of not voting. This is a direct result of Democrats in Congress and the Presidency doing things the base disagrees with or not doing things the base wants to see done. It appears politically stupid to act as they have, and yet, they did. So why?
Elected Democrats at the Federal level are members of the national elite. If they weren't a member when they were elected, they are quickly brought into the fold. They are surrounded by lobbyists, other members and staffers who were lobbyists, as a rule. They learn they need to raise immense amounts of money in the off years when normal people aren't giving, and that the only way to raise that money is for corporate interests and rich people to write them. They also receive the benefits of elite status, very quickly. It's not an accident that the every Senator except Bernie Sanders is wealthy.
Whatever Americans think, whether they support a public option or single payer; whether they're for or against Iraq or Afghanistan; whether they agree with bailing out banks or not, elite consensus is much much narrower than American public opinion. It starts at the center right and heads over to reactionary (repeal the entire progressive movement and the New Deal, taking America back to the 1890s).
The elites are convinced they know what has to be done. Not necessarily what's "best", but what is possible given the constraints they believe America operates under and the pressures which elected officials work with. So Obama can say, and mean, that if he were creating a medical system from scratch, he'd go with single payer. But he "knows" that's impossible, not just for political reasons, but because there are huge monied interests who would be horribly damaged or even destroyed by moving to single payer. On top of that, he looks at the amount of actual change required to shift all that money away from insurance companies and to reduce pharma profits, and to change which providers get paid what, and he sees it as immensely disruptive to the economy. In theory, it might lead to a better place, but to Obama, the disruption on the way there is unthinkable.
The same thing is true of the financial crisis. The banks may be technically insolvent, but the idea of nationalizing them all, or shutting them down and shifting the lending to other entities would mean that the most profitable (in theory, not in reality) sector of the economy would largely be wiped out. Add to that the fact that Obama was the largest recipient of Wall Street cash of the major candidates for the Presidency, and the immense influence the banks wield through their alumni who are placed throughout the Federal Reserve, the Treasury and other departments, and the idea of actually radically reforming the banking system becomes unthinkable. Virtually every technocrat giving Obama, or most Senators advice, will be against it.
Moreover they understand that with a few exceptions, the financial economy is the American economy. It's what the US sold to the rest of the world: pieces of paper in exchange for real money which could be used to import real goods, so Americans could live beyond their means.