Ironically, for all the talk of bipartisanship, the stimulus bill failed to get one vote from a House Republican. In fact, it was opposition to the stimulus bill that attracted bipartisan support, from both Republicans and Democrats. That's what happens when you draft an almost trillion dollar package in secret, by a select few, with no bipartisan input.
Having passed the bill, why would Pelosi et al "hail the measure as immediate" and "vow the bill will bring immediate relief"?
They have to know that they're setting expectations too high.