Matthew Poole and continuators on 2 Chronicles 30:17

17 For there were many in the congregation that were not sanctified : therefore the Levites had the charge of the killing of the passovers , for every one that was not clean , to sanctifie them unto the LORD.

That had more desire to come to the Passover, than care and diligence to cleanse and prepare themselves for it. Now these persons were either 1. The Priests, who were before taxed with Uncleanness and Unpreparedness. Or rather 2. the People, as is implied in the word Congregation, and expressed in the next verse, and confirmed by the Reason here following.

Because many of the People were unclean, and therefore many more Sacrifices were to be offered for their Purification than the Priests could kill.

i. e. Either 1. The Paschal Lambs; which the Levites here offered, because of the Impurity of many of the Masters of Families, who otherwise, as some think, should have killed the Paschal Lambs in their own Houses, Exod. 12. 3. Or, 2. The other Sacrifices which accompanied the Passover, which also are called by the same Name, as is evident from Deut. 16. 2, 3. where see my Notes: Which the Levites here killed, for the Reason now mentioned.

So far as they could make any discovery of their uncleanness either by their own Confession, or otherwise. But there were many whose Uncleannesses were not known, and therefore did not cleanse themselves, as is said, v. 18. as they ought to have done.