(It wasn't. Crucifixion was was unknown in Egypt at the time.)
Pharaoh said ... Surely I shall have your hands and feet cut off upon alternate sides. Then I shall crucify you every one. Quran 7:123-3
(Pharaoh) said ... I shall cut off your hands and your feet alternately, and I shall crucify you on the trunks of palm trees. 20:71
He [Joseph] said ... he will be crucified so that the birds will eat from his head. 12:37-41
(Which is strange, since about the only thing we know about Jesus is that he was crucified by the Romans.)
And because of their saying: We slew the Messiah, Jesus son of Mary, Allah's messenger - they slew him not nor crucified him, but it appeared so unto them; and lo! those who disagree concerning it are in doubt thereof; they have no knowledge thereof save pursuit of a conjecture; they slew him not for certain. 4:157