The problem with politics is that few people are objective. Liberals say everything is Bush's fault. Conservatives say everything is Obama's fault. The truth is that they all share in the blame. Bush started a big mess and Obama is making it bigger. The coin has two sides but I see very few people who are willing to see more than one.
Because Bush made the decision not to defend himself after leaving. Clinton defended his legacy vigorously; Bush senior also did not defend his legacy, and it was a good one. But Bush junior has been absolutely absent from the field. If he and his surrogates would mount a defense, you would see Obama's numbers going down, because he is leaving the impression he has no responsibility, when after 4 years, you just can't blame your predecessor anymore.
As with most things in life it has been a series of actions that have led us to our current predicament, it started 30 years ago. As such we cannot blame any one particular person for our woes, nor can we blame one person for not having it fixed in 4 years While he contributed to it the main reason Bush is blamed is that it happened under his watch, that is all.
Because bush is the first president in history to start a war without adding in a way to pay for it. He started TWO wars and rather than pay for either he instituted "temporary" tax cuts that actually had us in hot water by 2004. He then added in a Medicare prescription "benefit" that benefited pharmaceuticals more than anyone else and he didn't pay for that either. All the debt he put us in just got kicked down the road to Obama. Obama has added to debt but the original problem was never addressed.