Why was it necessary for the body of Jesus to be taken down before sunset?
And if a man have committed a sin worthy of death, and he be to be put to death, and thou hang him on a tree: His body shall not remain all night upon the tree, but thou shalt in any wise bury him that day; (for he that is hanged is accursed of God;) that thy land be not defiled, which the LORD thy God giveth thee for an inheritance. (Deuteronomy 21:22-23)
It was against Jewish law for him to remain on the cross overnight after he had died. If he had still been alive, then they could have left him until he died.