Because of their fear,
His parents said.
‘He is of age!
διὰ τοῦτο οἱ γονεῖς αὐτοῦ εἶπαν ὅτι Ἡλικίαν ἔχει, αὐτὸν ἐπερωτήσατε.
John uniquely indicated that the parents (οἱ γονεῖς αὐτοῦ) of this man born blind, because of their fear (διὰ τοῦτο), said (εἶπαν) that he was of age (ὅτι Ἡλικίαν ἔχει) or mature enough to ask him (αὐτὸν ἐπερωτήσατε). They were not going to speak out one way or another about Jesus. They only maintained that this man was their child and he was born blind. They knew nothing about what happened to give him sight. He had achieved the age where he could respond for himself. They told these questioners to go and ask him. They had nothing more to say about their son.