Because our elected representatives lie and lie and lie and lie and lie.
And because the President has proven, even to his supporters, that he cannot be trusted. He promised to support gay marriage, an end to Clinton's "don't ask, don't tell", an end to extraordinary rendition, no lobbyists in his administration, transparency in legislation and administration, lower taxes if you make under $250K, racial healing, rejecting pork "line by line," to control spending, that unemployment would not exceed 8% if his stimulus was passed, that he would not duplicate Dubya's "signing statement" output, that he would never accept a nuclear Iran, that the stimulus was designed to be spent quickly and he consistently avers that Joe Biden is not a fool.
How can he be trusted on his promises regarding health care? Check this for an easy comparison between word and word, and hope there is never a deed. The only surprising part of that link is how Obama can so breezily tell an abysmally stupid lie, just after telling the rest of us to stop talking.
What must he think of us?