GDP has been
trending down since the 80's. So there are probably systemic reasons for this, but the proximate cause is still the credit crisis. Financial/credit recessions are fundamentally different from a 'normal' recession because they call into question the trust people have in the system. As a result, both people and companies are less willing borrow and more willing to save.
W/o the sort of spending we normally get coming out of a recession, there is no multiplier effect, money velocity stagnates and you anemic growth. But this is "normal" and the reason why 8 years later we are still in the midst of a "recovery."
But another issue has been the fact that the US govt hasn't done jack to stimulate the economy. The way fiscal policy is supposed to work is that you save money during the booms and spend during the busts. Spending, whether govt, business or individual is what causes the money supply to increase (via borrowing).
But since the crisis, the only increases in the MS have been from the fed. And that doesn't work if people don't take the money they get from selling assets to the fed and turnaround and spend it.
Just take a look at the money
multiplier and
velocity.