Matthew Poole and continuators on Numbers 19:22

22 And whatsoever the unclean person toucheth, shall be unclean: and the soul that toucheth it, shall be unclean until even .

Not he who is so onely by touching the water of separation, ver. 21. but he who is so by the greater sort of uncleanness, which lasted seven days, of which ver. 11, 16. and which was not removed without the use of this water of purification, as is manifest from the context and other places of Scripture, and from the nature of the thing: For the lesser sort of uncleanness, mentioned in the foregoing verse, lasted onely till even, and was cleansed by the sole washing of his clothes and flesh in water, ver. 7. without any use of this water of purification, which, if it had been necessary, must have been used on the third, and on the seventh day, according to the rule, ver. 12, 19. and so the uncleanness of one day had been made an uncleanness of seven days, which is a contradiction. Besides it is unreasonable, that he who immediately touched the defiling thing should be no more and longer unclean, than he who touched that person onely; and it is contrary to other rules in like cases, as Levit. 15. where the Man or Woman having an issue are thereby made unclean for seven days, ver. 13, 28. but he who toucheth them is made unclean onely till even, ver. 7, 27. And therefore this cannot be meant of him who was unclean by touching this water, who himself was unclean onely till even, ver. 21. as also he who toucheth him is in this place.

To signifie to us, the very infectious nature of sin and of sinfull company.

Because as his defilement was less, so it was fit the duration of it should be shorter.