Why did Benjamin Franklin change Thomas Jefferson's original phase of "We hold these truths to be sacred" to "We hold these truths to be self-evident"?

That is because using the word sacred has a religious connotation while self-evident has one based on rationality. It was based on Locke and Hume who claimed that truths that are self-evident are easily understood by anyone reasonable and that this is one such truth because it is rational and reasonable.