Why?
Because both the citizen parent and the foreign-born child complied with the terms of a naturalization statute.
It is true that the ‘Naturalization Act’ statutory conditionalities were met. However, that does not answer your question fully.
First point: Ted Cruz would have been a citizen of the Untied States absent the act due to natural inheritance of nationality from his mother under the theories set forth by Vattel and others that link citizenship to lineage, not to place, despite all the FUD and confusion thrown at this issue over the past eight years.
Second point: It is within the power of the legislatures (and Congress) to define the conditionalities and rules for inclusion in certain classes under certain words that may be found in the Constitution. For example, the Constitution mentions ‘resident’ in several places, but does not define the term. The determination of who is and who is not a ‘resident’ is left to the States generally, and their legislatures set the rules governing that determination by enacting statutes.
Congress is similarly empowered to define the conditionalities that must be met to acquire citizenship by birth, and Congress has done so. Your argument that this is a form of naturalization relies on the fact that the provisions are found in an act with the word nationalization in its title, but that is entirely non-determinative. The statutory requirements for citizenship by birth could just as easily have been found in an act called the “National Sagebrush Preservation Act,” but that would not mean that citizens by birth are therefore ‘sagebrush’.
Bottom line is that there is no cogent, thoughtful, rational, sensible reason to distinguish among citizens by birth; according ‘natural born’ status to some and not to all.