What is the wisdom behind the absence of an explicit text in the Quran and Sunnah stipulating the obligation of the niqab, like prayer, fasting, and zakat, despite the established evidence for its obligatoriness?

The fatwa given is the obligation for a woman to cover her entire body. The obligation of the niqab (face veil) does not require an explicit text with that exact wording; rather, it is sufficient for a sound دليل (proof/evidence) to indicate the obligation of covering the face. Even according to the view that the niqab is not obligatory, covering the face is better for complete protection and to emulate the Mothers of the Believers. It is incumbent upon the accountable individual to strive against their self in performing acts of obedience and opposing whims.