The reason was that there were American citizens living abroad at the time, some even in the diplomatic service, and this exception gave them time to adjust to the new realities of the newly adopted Constitution.
The 1790 Act created a statutory exception to the rule or definition that everyone knew to be so, thus the language:
"the children of citizens of the United States that may be born beyond sea, or out of the limits of the United States, SHALL BE CONSIDERED AS as natural born citizens".
Not "are" but "shall be considered as". It's an exception -- not a definition. The exception ended in 1795.
These words from Justice Waite in Minor vs Happersett on the matter:
“The Constitution does not, in words, say who shall be natural-born citizens. Resort must be had elsewhere to ascertain that. At common-law, with the nomenclature of which the framers of the Constitution were familiar, it was never doubted that all children born in a country of parents who were its citizens became themselves, upon their birth, citizens also. These were natives, or natural-born citizens, as distinguished from aliens or foreigners. Some authorities go further and include as citizens children born within the jurisdiction without reference to the citizenship of their parents. As to this class there have been doubts, but never as to the first. For the purposes of this case it is not necessary to solve these doubts. It is sufficient for everything we have now to consider that all children born of citizen parents within the jurisdiction are themselves citizens.“
Congress has no authority to make an exception to a Constutional requirement. The 1790 act was drafted by the founders, and they of all people knew they had no authority to unilaterally grant an exception to the NBC clause. What the founders did in 1790 was to further define the archaic language to reflect their understanding of the meaning of the term at the time the constitution was drafted BY THEM.
George Washington knew that congress had no authority to grant an exception to the Constitution and he signed the act. The 1790 act CLARIFED the founders intent in drafting that clause.
To insist that the First Congress intended to grant an exception to the Constitution is to claim that one of the First acts of the founders was to violate the Constitution and that George Washington gleefully went along with that illegal act.
Who should I trust on this issue? You or George Washington?
Tell me Uncle Chip, is this a deal breaker for you, or are you willing to give Ted Cruz the benefit of your doubt in an attempt to save the Republic?