A.T. Robertson Commentary Mark 9:28

"And when he was come into the house, his disciples asked him privately, [How is it] that we could not cast it out?" — Mark 9:28 (ASV)

Privately, saying (κατ' ιδιαν οτ). Indoors the nine disciples seek an explanation for their colossal failure. They had cast out demons and wrought cures before. The Revisers are here puzzled over Mark's use of οτ as an interrogative particle meaning

why where Mt 17:19 has δια τ. Some of the manuscripts have δια τ here in Mr 9:28 as all do in Mt 17:19. See also Mr 2:16 and 9:11. It is probable that in these examples οτ really means

why . See Robertson, Grammar, p. 730. The use of ος as interrogative "is by no means rare in the late Greek" (Deissmann, Light from the Ancient East, p. 126).