The shadow government that controls both parties makes equal use of Democrats and Republicans. They need to continue to show some small degree of difference between the parties, to maintain the illusion of choice on election day. These small differences are played up as much as possible, while the huge areas of agreement are ignored.
People with a conscience generally see the Dems as the good guys and the Repubs as the bad guys. This is a particularly useful fiction at this point in history because so many progressives were distraught to the point of despair about the Bush administration. Some reason needed to be held out for hope, and indeed Obama's main theme was hope.
Millions of deluded Obama supporters wasted their energy on insuring Obama's election rather than working for true change at the top (that layer over the visible government). Millions more were pacified that change was on its way, and thus took no meaningful action. If throngs of citizens had descended upon Washington to protest: