It is my understanding that they use "Nuasch Sefharad" due to 18th century Rabbi Yosef ben Moshe Mammon (Maimon) Maravi deciding to move to Bhukara and teach them in the face of severly declining observence, wikipedia description here https://en.m.wikipedia.org/wiki/Yosef_Maimon

It is my understanding that they use "Nuasch Sepharad" due to 18th century Rabbi Yosef ben Moshe Mammon (Maimon) Maravi deciding to move to Bhukara (from his native Morocco) and teach them in the face of severly declining observence, wikipedia description here https://en.m.wikipedia.org/wiki/Yosef_Maimon