Albert Barnes on Hebrews 8:4

For if he were on earth, he should not be a Priest. He could not perform that office. The design of this is to show a reason why he was removed to heaven. The reason was, that on earth there were those who were set apart to that office, and that he, not being of the same tribe with them, could not officiate as priest, There was an order of men here on earth consecrated already to that office, and hence it was necessary that the Lord Jesus, in performing the functions of the office, should be removed to another sphere.

(1) "that there" "they"