Albert Barnes Commentary 2 Chronicles 29:34

"But the priests were too few, so that they could not flay all the burnt-offerings: wherefore their brethren the Levites did help them, till the work was ended, and until the priests had sanctified themselves; for the Levites were more upright in heart to sanctify themselves than the priests." — 2 Chronicles 29:34 (ASV)

The Levites were more upright. The high priest Urijah, for example, had participated to some extent in the impieties of Ahaz (2 Kings 16:10–16). He and many of the priests may, therefore, have looked coldly on the reforming zeal of Hezekiah.