Albert Barnes Commentary Numbers 19:1

"And Jehovah spake unto Moses and unto Aaron, saying," — Numbers 19:1 (ASV)

The principle that death and everything related to it, as the manifestation and result of sin (Genesis 2:17), are defiling, and thus lead to interruption of the living relationship between God and His people, is not introduced now for the first time, nor is it at all unique to the Mosaic law.

On the contrary, it was traditional among the Israelites from the earliest times; it is assumed in various enactments already made (Numbers 9:6 and following; Leviticus 10:1, 7; Leviticus 11:8, 11, 24; Leviticus 21:1 and following), and it is traceable in various forms among many nations, both ancient and modern.

Moses, here as elsewhere, adopted existing and ancient customs, with significant additions, as aids in the spiritual education of his people.

The ordinance was probably given at this time because the plague that occurred (Numbers 16:46–50) concerning the matter of Korah had spread the defilement of death so widely through the camp that it seemed to require special measures of purification, especially since the deaths resulting from it were, in an extraordinary way, the penalty of sin.