One plausible explanation is this.
Early American colonies tended to define themselves using England as a negative bench mark. England was a very hierarchical society which placed great value on old families. Old documents do not mention first name, they routinely refer to a person by his family name as if any one who mattered would know who was meant.
What became USA advisedly decided to do away with emphasis on pedigree. If you address a person by his first name, you are making an important statement:You get no credit for what your ancestors, brothers or cousins did. What matters is what your own credentials are.