It doesn't need to, because it is axiomatic. You want to create qualifications that certain persons cannot meet, in order to specifically exclude them, but those qualifications are just that; Created. Made up. Without intrinsic value. Subjective. Arbitrary.
It's interesting that you should refer to that particular one, since it was specifically written to address lingering citizenship and civil rights questions regarding emancipated slaves, who were widely regarded as "not" or "less than" human by the left side of that debate. Ironically, using much the same kind of arbitrary and subjective criteria currently used to defend abortion.
