Al-qanitin presents Maryam as belonging to a broader category identified by devout obedience. A formulation confined to a female-only category would convey a different scope; the existing wording places a female individual among the obedient as a larger class.
The rhetorical effect includes inclusion in an honored group, rather than isolated praise of an individual. The source wording remains al-qanitin; comparison with other possible formulations serves only to clarify the meaning.