Why were Nega'im in garments taught in between Nig'ei Adam and Taharah from Tzara'as?
Hadar Zekenim (46, citing Bechor Shor): It is because there cannot be a Metzora who need not immerse his clothes, therefore it taught their Taharah together.
Why does it say "the garment in which there will be a Nega" [and not 'when there will be a Nega in a garment']?
Moshav Zekenim: This alludes to the Isur of wearing haughty clothes of Nochrim. Their priests have a symbol of idolatry on their garments.