The answer is that because they recognized in the 50s (perhpas relearned or rediscovered is a better term) that this nation was founded on those principles and has everything to do with, from its founding, the idea that there is a God in Heaven, regardless of persuasion, that watches over the affairs of man, and who endows upon mankind their unalienable rights.
Without that knowledge, it is impossible to understand the basic founding principles of this Republic and the resulting civic responsibility. John Adams said it best:
"We have no government armed with power capable of contending with human passions unbridled by morality and religion. Avarice, ambition, revenge, or gallantry, would break the strongest cords of our Constitution as a whale goes through a net. Our Constitution was made only for a moral and religious people. It is wholly inadequate for the government of any other."- John Adams, Oct. 11, 179
Which one would you raise to preeminence over all others as a basis for national morality?