Several reasons - the largest being that the founding fathers had nothing to do with that line. It, along with the "In God We Trust" on our money was added in the 1950's when religious zealots insisted on it. The founding fathers declared there would be complete separation of church and state. It irks me to no end when I hear people say, "But that's not really what they meant." These were some of the most brilliant minds of the age who wrote these lines. Are you trying to tell me they were unable to articulate their true meaning?! No, they meant exactly what they said, and for good reason. So whenever I hear religion mentioned in matters of state (including election campaigns) I get very angry. This country was not founded on religion, but on freedom.
Freedom of religion includes freedom from religion. I don't have an option in currency, and I shouldn't have to see a "we" that includes me proclaiming a trust in a myth. My children should not have to stand in a public school and utter an oath which includes religion in order to profess a love for country. "One nation, indivisible, with liberty and justice for all."