'Rashi': They had delayed becoming Tahor. This is why they delayed coming until now. They did not believe [that they would serve Hashem in the Mikdash], for Achaz abolished [Avodas] Kehunah. Is it possible that Achaz served idolatry, and his son Chizkiyah, in his first year, in his first month, will tell the Kohanim and Leviyim to serve Hashem alone, and he will need us [to serve]?! Therefore, they slackened from coming. When they investigated, they found that it was totally l'Shem Shamayim, all sanctified themselves.