Moshav Zekenim citing R. Yehudah ha'Chasid: It is because they needed to write Hashem's name, and they needed 10 to oversee him, to have intent together. From here we learn that one who writes names of Hashem in a Sefer Torah, Tefilin or Mezuzah must do so in front of 10 [men] and immerse [beforehand] and be in a place of Taharah, for a matter of Kedushah cannot be in front of less than 10.