Paul Krugman poses the titular question and explores some possibilities, among which are "an inadequate plan combined with a wildly overoptimistic forecast [that] was more or less guaranteed to create the impression of a failed program." "Wildly overoptimistic" is far too generous to the president.
Obama sold the stimulus package in large part on the claim that there were, as NPR put it in February 2009, "thousands of projects nationwide that he calls "shovel ready" — meaning planning is complete, approvals are secured and people could be put to work right away once funding is in place."
We now know, of course, that this was a lie. Or, to give Obama the benefit of the doubt, at the very least a negligent misrepresentation.
Even Obama now admits that there were no shovel ready jobs.
The whole concept of "stimulus" was thus tainted by the lies that were told to sell the last one. To cite an appropriate aphorism, my guess is that the American people are thinking "fool me once, shame on you, fool me twice, shame on me."