- This was a declaration of loyalty by the Shevatim on Yaakov's deathbed, so it's a worthwhile prayer. But since it's not a verse found in the Torah, we say it quietly (Pesachim 56a)
- This was a prayer of the angels so it's not appropriate for sinful mortals to say it aloud, except for on Yom Kippur
From the Artscroll Yom Kippur Machzor, pp.69-70