Hebrews 8:4 - Albert Barnes' Notes on the Bible

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 people 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.