Albert Barnes Commentary Hebrews 7:23

"And they indeed have been made priests many in number, because that by death they are hindered from continuing:" — Hebrews 7:23 (ASV)

And they truly. Under the Jewish dispensation. The purpose of this verse, and the one that follows, is to state one more reason for the excellence of Christ's priesthood. The reason is that, due to the frailty of human nature and the shortness of life, the office of priest under that system was continually changing. But here there was no such change. Christ, being exalted to the heavens to live forever there, has now an unchangeable priesthood, and everything concerning his office is permanent.