Where exactly does it say that was required?
Natural Born Citizen relates to an earlier concept of Natural Born Subject, as colonists they were not citizens but subjects. So they didn’t have to spell it out the way later folks might have felt or feel the need to.
Especially as they may be like Obozo and want to rip it to shreds.
NBC however is distinct from NBS because it is mentioned in relation to who can be the chief executive/administrator, the closest we Yanks get to who can be the king for Britain. Thus it has a particular added weight when compared to NBS that normally would not touch on their sovereign question.
To be born only with one allegiance is the heart of NBC.
If there were any reasoning that only one parent was needed it would be a father who could confer it, not a mother. That case too would arise from how NBC compared to NBS.