Nicholas Lemann has a plausible theory as to why Obama's shied away from bank nationalization and a thorough overhaul of Wall Street:
But it isn’t difficult to guess why the President chose instead to endorse the new phase of the financial rescue plan that Treasury Secretary Timothy Geithner unveiled last week. Obama surely did not decide to run for President because he was burning with a passion to reform the financial system. He has to address the crisis, and he is trying to add enough new controls to the system to prevent a repeat of it, but it looks as if his heart is with the big new programs in his budget and with his foreign-policy initiatives.
Bank nationalization would drive the stock market down and increase the agita of people with 401(k) plans. Moderate Democrats in Congress would further soften in their support for the Administration’s legislation. The price of bank nationalization might be Obama’s super-ambitious plans in other realms, which, if history is a guide, are likely to pass only in this first year of his Presidency. If they do pass, he will have generated tax revenues from affluent people for social purposes far beyond those of the House’s tax on A.I.G. bonuses, and he will have significantly eased the distress of people who can’t get good health care or education. That is a lot to put at risk.
Not sure whether avoiding fixing the banking industry because it doesn't fit with the plans he laid out for his presidency prior to taking office is a good strategy, but it wouldn't surprise me if Obama's thinking is not that far off from what Lemann lays out here.