Well Daniel Larison has a very interesting explanation for why our President hasn't been following his M.O. with the Treasury Secretary.
Very plainly, Obama does not protect his friends, but he will promote and defend those whom the establishment embraces and approves. On the contrary, it is his friends, mentors and close associates who seem to be among the most expendable, and they are the ones he tends to drop when they become liabilities. Allies and supporters who continue to be useful are retained. ...Read the whole post, if you have time. It's quite good.
Geithner remains one of the main public faces of the failed establishment that Obama has spent his national career accommodating, and to do anything other than stand by one of his senior Cabinet members this early in his administration would mean political disaster for him. Obama has become trapped by his own accommodationist instincts, as the policies and personnel that the establishment embraces prove to be deeply flawed. Obama does not tend to break with establishment conventions, which significantly limits his ability to break with figures such as Geithner after they have already received the establishment seal of approval.