I suspect this fact is rather instructive.
"I don't want to jeopardize my legacy" would probably be a good ballpark answer. Rumsfeld needed to be replaced with Card and Powell. The only reason not to do so, considering how poorly our efforts Iraq and Afghanistan are doing, is because of the fears that it would be seen as evidence that Iraq and Afghanistan aren't doing well. When your PR angle is 'The sun will come up tomorrow', it's downright dangerous to the message to make moves to the contrary.
Probably more on the money that you think.
I don't buy into the garbage about things going poorly in Iraq. We are still killing insurgents by the bushel, Al Qaeda is under the microscope and can't transfer people, weapons, or money without us knowing about it, or without them using the Russians and Chinese to do it.
The government is there, its working, the GDP and infrastructure are improving, and most of the country is stable.
We were in Germany for 44 years after WWII! We're STILL in Japan, and I believe we are legally responsible for defending her at this late date.
Our being there in those kinds of numbers is kind of handy with Iran threatening regional nuclear war against Israel.
I guess a lie repeated enough becomes the truth quicker than anyone realizes.