Charles Ellicott Commentary Exodus 30:20

"when they go into the tent of meeting, they shall wash with water, that they die not; or when they come near to the altar to minister, to burn an offering made by fire unto Jehovah." — Exodus 30:20 (ASV)

That they die not. —Compare to Exodus 28:35; Exodus 28:43. It is not altogether easy to see why the death-penalty was threatened against neglect of certain ceremonial observances, and not of others. Ablution, however, was so easy, and probably so long-established a practice, that to omit it would imply intentional disrespect towards God.