Albert Barnes on Matthew 17:10-13

See also Mk 9:11-13. Why then say the Scribes, etc. The disciples appear to have been satisfied now, that he was the Messiah. The transfiguration had taken away all their doubts. But they recollected that it was a common doctrine among the Jews that Elijah should appear before the Messiah came; and they did not then recollect that he had appeared. To this difficulty the word then refers. "We are satisfied that thou art the Christ. But Elijah is not yet come, as was expected. What, 'then,' is the meaning of the common opinions of our learned men, the scribes? Were they right or wrong in their expectation of Elijah?" Mt 11:14.

(n) "Why then" Mal 4:5,6, Mt 11:14