The reason is that as per the awakening process model found in suttas like AN9.34, AN9.38, AN9.39, AN9.40, AN9.44, and AN9.47 the non-provisory nibbana (nippariyāyenā nibbānaṃ) is to be understood as something that comes with the attainment and transcending of the dimension of neither perception nor non-perception (nevasaññānāsaññāyatanaṃ).
I have tried to investigate this subject further in the topic:
