This omission of the definite article the (beginning in the 1837 edition) is probably a typo rather than the result of a conscious editorial change. There seems to be no motivation for such a change, nor was it marked by Joseph Smith in the printer’s manuscript. In fact, nearby in 2 Nephi 29:7, we have the same basic phraseology in “yea even upon all the nations of the earth”, so this 1837 change seems very much like a typo. Elsewhere in the text, we have five occurrences of “all the nations of the Gentiles” as well as one occurrence of “all the nations and kindreds of the earth” (in 1 Nephi 14:15). Except for this accidental error in 2 Nephi 29:12, we have no examples of “all nations” postmodified by a prepositional phrase headed by of.
Summary: Restore the definite article the in 2 Nephi 29:12 so that it reads according to the earliest text: “all the nations of the earth”.