Steve Benen states the answer:
...for all the talk in far-right circles about Bush not being conservative enough, some of his most painful disasters came because he refused to stray from his conservative ideas.Bush will go to his grave with two mistaken beliefs. First, he still believes that God appointed him President to do God's will. Second, he will have to believe that his decisions were God-inspired and correct in the long run. He will never accept the fact that the current recognition of his utter failure will be the one that historians tag him with in the far future.
This is probably a little too casual an analysis, but it seems this touches on one of the more glaring differences between Bush and Reagan -- both instinctually backed conservative ideas driven entirely by far-right ideology, but Reagan reversed course when those ideas failed. Bush didn't.
When Reagan's tax cuts didn't work, he reversed course and approved significant tax increases (several times). When Reagan's antagonism towards the Soviets didn't work, he reversed course and compromised on arms control.
But Bush, with very few exceptions, could ever own up to his errors.
Right now, he has nothing to lose by accepting a stimulus package, except his ideological pride. So it doesn't happen, no matter how much it might help. His approach to the economy has been a spectacular failure, and when given a chance to go in a different direction, Bush has decided on a legacy of consistency, instead of success.
How could he be wrong? He is a conservative who has accepted Jesus Christ as his savior and he has applied conservative principles consistently.