The stimulus was small

At least when compared to the competition:

This paper studies the patterns of fiscal stimuli in the OECD countries propagated by the global crisis. Overall, we find that the USA net fiscal stimulus was modest relative to peers, despite it being the epicenter of the crisis, and having access to relatively cheap funding of its twin deficits. The USA is ranked at the bottom third in terms of the rate of expansion of the consolidated government consumption and investment of the 28 countries in sample.

On the same point, Paul Krugman posts this graph looking at total public spending -- that is to say, federal spending, which went up, but also state and local spending, which went down -- in recent years:

"Looking at this graph," Krugman asks, "if you didn't know there had been a 'massive' stimulus, would you even have suspected that there had been any stimulus at all?"