I wouldn't bet a plugged nickel on it, but I don't think there was any necessity for re-naming the NHRA Nationals until 1961 because there was only that one race, annually. No chance for confusion, nor any reason to separate it from other races. The Dertroit race was the only national event NHRA held that year.
However, in 1961, the inaugural Winternationals was held at Pomona, and now it would be necessary to identify the Indy race as something other than just "The Nationals," since there were now, two "Nationals."
Hence, I believe, the "U.S. Nationals" moniker was born.
Could be wrong, but it makes sense to me.
Uh oh... I forgot; this is NHRA!!!