Oznayim la'Torah: Because in all other cases, where a number of Kohanim carried the limbs to the Mizbe'ach, and where one Kohen therefore carried the head alone, it was necessary to cover the Beis-ha'Shechitah with the fat; whereas here, where Moshe had to the perform the Avodah on his own, he probably took a number of other limbs together with the head, in which case it was not necessary to cover the Beis ha'Shechitah. ¹