1. Early in Bush's first term there were a whole bunch of scandals on Wall Street. Big brokerage houses were found to be abusing their customers' trust, selling them stocks the brokerage knew were junk, then betting against them. There were stock price manipulation scandals, insider trading scandals, etc. etc. People looked to The President to remedy this situation but the Republicans don't believe in regulation, they believe stock brokerages should be able to regulate themselves. So the problems got worse and worse. Bush defunded the SEC (the Securities Exchange Commission, set up by FDR after the last great stock market crash) so they weren't really able to do their job.
2. Regulations were also relaxed on banks. Bush and the Republicans repealed the Glass Stegal act, another law passed in the wake of the last crash to separate investment and deposit banking, which allowed big banks to basically gamble with their depositors' money. Banks were allowed to package mortgages together and sell them to investors, then they'd get their money back and lend it out again, so they could use the same money over and over. This basically made deposit banks into investment brokers. And since banks weren't lending THEIR OWN money, they lowered their standards for who they would lend to. This worked great as long as real estate values were climbing rapidly, but as soon as the went down, even the tiniest bit, the whole system collapsed, and property values crashed around the country.
3. Bush came to office with the budget nearly balanced. That first year, if he'd done nothing, if he'd just sat on his hands, he would have had a balanced budget, and he could have taken credit for it. Instead he simply could not wait to undo all the good work Clinton had done, he gave away the prospective surplus in a big tax cut for the rich. Bush ended up doubling the whole pre-existing national debt, and that's NOT COUNTING the two wars, which he financed on the credit card. I think those 54% of people finally realized that every Republican president since Reagan has promised to make the budget his top priority but once in office decided debt simply didn't matter.
4. Despite what you hear from Rush Limbaugh, our economy has improved steadily under Obama. In fact for the last century or so the Democrats have done much better on economy issues than the Republicans.
5. The simple matter of taking responsibility. McCain and Romney were not even able to distance themselves from Bush's policies, to admit even that he'd made 'honest mistakes' and tell us what they would do differently. In fact the both of them seemed to advocate a return to Bush's policies, to Reaganomics, even though it's been a total disaster for 95% of us for 30 years. As for Bush himself, he never actually said "Whoops, we did something wrong. Mistake were made. We should do this other thing now." He acted as if nothing bad happened! I think this is the main reason people blame him.