Again, you're assuming that they have, indeed failed. While it's true that they may appear to have failed over the short term, the long term effects of his presidency have yet to be known.
Abraham Lincoln was considered a terrible president at the time. His significance to history wasn't realized until long afterwards.
Not to say that Bush can be compared to Lincoln. No one would be more surprised than I if he were seen to be anything other than a poor, if not downright bad, president.