Albert Barnes on Matthew 17:19

Then came the disciples, etc. This inquiry was made in some house to which they retired near the place where the miracle was performed, (Mark) Jesus told them, in reply, that it was because of their unbelief that they had not been able to cast him out. They were appalled by the difficulty of the case, and the obstinacy of the disease. Their faith would not have made it more easy for God to work this miracle, but such was his will; such the way in which he worked miracles, that he required faith in those who were the instruments.