The Rambam in Hilchot Yom Tov (7,13) says that on Hol Hamoed it is Assur to write Tefilin or Mezuzah for others unless the Sofer needs the money for Tzorech Yom-Tov. Kesef Mishneh says that the Rambam does not count the writing of the Tefilin itself as a Tzorech Yom-Tov, because the Rambam holds that one does not put on Tefillin during Hol Hamoed. But Mezuzah surely is something that is needed on Hol Hamoed for someone who doesn't have a mezuzah!
Why isn't writing a Mezuzah for others a Tzorech of the Moed?