The Lord gave Aaron and his sons a privilege that no one else could enjoy,
“So you shall appoint Aaron and his sons, and they shall attend to their priesthood; but the outsider who comes near shall be put to death” (Numbers 3:10).
Only Aaron and his sons could serve as high priests, and truly, they had to fulfill that obligation, for it was their lot in life, and no one else on earth could do what they did, especially entering into the Holy Place and then the Holy of Holies.
Why did God make it so exclusive that if anyone else tried to do what they did, the violator paid for it with his life?
I do not know, but throughout the world and throughout biblical teachings, He has designated that certain people do certain things, and that is His prerogative, for He is the Creator, and it is our job to honor Him by doing as He wants us to do, and by not attempting what He has not authorized us to do.