Albert Barnes on Hebrews 7:23

And they truly. Under the Jewish dispensation. The object of this verse, and the following, is to state one more reason of the excellence of the priesthood of Christ. It is that, owing to the frailty of human nature and the shortness of life, the office of priest there was continually changing. But here there was no such change. Christ, being exalted to the heavens to live for ever there, has now an unchangeable priesthood, and everything in regard to his office is permanent.