NOTES for Lev 21:1-24
For priests, the norms and rules of ritual purity were always especially strict. This is understandable: after all, these were people who, throughout their service, remained at the altar, not leaving the court of the Tabernacle or, later, the Temple. For such people, all concessions connected with preserving purity were ideally excluded completely, and in reality were reduced to a minimum. In a fallen world, it is impossible for a person to avoid impurity completely. Impurity in it is a consequence of the fall. God, of course, did not create anything impure, and for Him the world remains pure just as it remains the Kingdom for Him.
Impurity exists for us and for our world insofar as we have separated ourselves from God, separating from the fullness of the Kingdom that part of creation which we managed to draw after ourselves. This separation is what gives rise to impurity in the world; it is the consequence of the world's brokenness and its falling away from God. Its forms are varied, but all of them are, in one way or another, manifestations of the death that entered the world together with sin. A person carries this death within himself, and therefore sins, actualizing within himself what deprives him of life. So it has always been, and so it will be until human nature is completely transformed; but before the coming of Christ, a person was also strongly affected by the impurity of the world, since it was characteristic of the world.
This effect was added to the action in a person of his own sin, which was the main source of defilement for him. For a person whose service required a state of consecration, it was absolutely necessary to minimize every defilement, both what comes from his own sin and what is connected with the influence of external impurity. Hence the stricter rules of ritual purity for priests: they could not become defiled even temporarily, even in cases where that was acceptable for others.
That is why a physically impaired person could not carry out priestly service: every such impairment was perceived as a manifestation of the same uncleanness present in the world. The person, of course, was in no way guilty of having experienced its effect, but it nevertheless made him unfit for service, just as one or another physical limitation can make a person unfit for some specialized work requiring perfect health or certain special physical abilities.