He didn't have to. It's common knowledge that several states did not recognize children of aliens to be citizens. For example, Virginia law in 1779 deemed infants wheresoever born to non-citizens to be aliens.
The U.S. didn't even have a birthright citizenship law until 1866, which clearly states: "all persons born in the United States and not subject to any foreign power, excluding Indians not taxed, are hereby declared to be citizens of the United States"
We know for a fact the intent and meaning of the 14th Amendment's 'subject to the jurisdiction' requirement because Senate Judiciary Committee Chairman Trumbull, who played a pivotal role in the passage of the 14th Amendment according to the U.S. Senate's own history, declared it as such: "the provision is, that 'all persons born in the United States, and subject to the jurisdiction thereof, are citizens.' That means 'subject to the complete jurisdiction thereof.' What do we mean by 'complete jurisdiction thereof?' Not owing allegiance to anybody else. That is what it means."
Like so many other things claimed by those bent on twisting the Constitution, this is simply not true.
Fallacy: A 1779 citizenship law authored by Thomas Jefferson implies people born in Virginia had to have citizen parents in order to be citizens.
Truth: Jefferson's citizenship law, applied to every person born in Virginia, was straight law-of-the-soil. All persons born in Virginia were citizens of the Commonwealth, without regard to any citizenship requirement of their parents.
Here's the part that's relevant:
Be it enacted by the General Assembly,
that all white persons born within the territory of this commonwealth
and all who have resided therein two years next before the passing of this act,
and all who shall hereafter migrate into the same; and shall before any court of record give satisfactory proof by their own oath or affirmation, that they intend to reside therein, and moreover shall give assurance of fidelity to the commonwealth;
and all infants wheresoever born, whose father, if living, or otherwise, whose mother was, a citizen at the time of their birth, or who migrate hither, their father, if living, or otherwise their mother becoming a citizen, or who migrate hither without father or mother,
shall be deemed citizens of this commonwealth, until they relinquish that character in manner as herein after expressed:
And all others not being citizens of any the United States of America, shall be deemed aliens.
So Jefferson lists a bunch of categories of people who all get to be citizens. Let's take out all the superfluous categories and cut to the chase:
Be it enacted by the General Assembly,
that all white persons born within the territory of this commonwealth...
shall be deemed citizens of this commonwealth, until they relinquish that character in manner as herein after expressed.
Now have I done any violence to this text? No, I have not. I just removed a bunch of categories to focus on the one category that's really relevant.
Jefferson says ALL WHITE PERSONS BORN IN VIRGINIA get to be citizens of Virginia.
This is STRAIGHT JUS SOLI. Absolutely no citizen parents required. ALL WHITE PERSONS born in Virginia were citizens.