it's all complicated, but when a president presides over the 8 year runup to the worst financial crisis in 70+ years...he has to be responsible at some major level. The deregulatory bent of the R party, who Bush2 put in charge of the fed and other orgs, the lack of stepping in when he should have...
I mean it was 8 years. Bush was president for the 8 years before the crisis. it's not like he stepped in just before it happened.
For ex I don't blame Bush for 911 - it happened early, it wasn't really preventable by the president. But the fianncial crisis is different and he gets blame for it.
but like I said - the main culprit is basic greed of bankers and home owners.