They were not able
Because Isaiah also said so.”
διὰ τοῦτο οὐκ ἠδύναντο πιστεύειν, ὅτι πάλιν εἶπεν Ἡσαΐας
Then John uniquely indicated that they were not able (οὐκ ἠδύναντο) to believe (πιστεύειν) in Jesus because Isaiah also had again said (ὅτι πάλιν εἶπεν Ἡσαΐας) and predicted this (διὰ τοῦτο). John was blaming the lack of belief in Jesus by the Jerusalem Jews because of the fulfillment of the prophetic statements of Isaiah, chapter 53:1, about the unbelieving Israelites at his time. Thus, they were like them, unable to believe in Jesus. Why don’t more people believe in Jesus?