1)
Why does it say "Ko Amar Hashem"?
1.
Radak: He should tell the Kohanim that Hashem told him to ask them.
2)
Why should he ask the Kohanim?
1.
Rashi: Perhaps during the Galus they forgot the laws of Kodesh, Tum'ah and Taharah.
2.
Malbim: This will clarify to them that they still did not purify and sanctify themselves enough, until Divine [influence] will take effect on the Bayis and nation in the end of days.
