Because to do otherwise would mean admitting he was wrong:
Q: If you had to do it over again, knowing what you know now, would you support the surge?
A: No, because, keep in mind that…
Q: You wouldn’t?
A: Keep in mind… These kinds of hypotheticals are very difficult. Hindsight is 20/20. But I think that, what I’m absolutely convinced of, is that at that time we had to change the political debate because the view of the Bush administration at that time was one that I just disagreed with.