(a) Excellent question! It seems that the Metzora nevertheless wants to be examined by the Kohen, and he comes to the Kohen after his days of counting in order to have the Kohen examine him and (hopefully) declare him to be Tahor. Certainly he does not want to be required to stay out of normal society for all of his life, and it is worthwhile for him to have to separate from his wife for seven days in order to become Tahor so that he can be allowed back into normal society.

However, with regard to Chol ha'Mo'ed, we do not want to cause him even temporary suffering, even though its purpose will be positive for him (since he will not reach this positive objective of becoming Tahor until after the festival, and during the festival he will only experience Tza'ar). We find a similar thing earlier (8b-9b) with regard to the Isur for a woman to use cosmetic plaster upon her face during Chol ha'Mo'ed, since it causes discomfort for her, even though it will be a source of pleasure for her after the festival.

(b) When Uziyahu became a Metzora, it was evident in the eyes of all the people, and he was put into Hesger, then declared a Metzora Muchlat, and sent outside of the city. Under those circumstances, it was not possible for him to be alone with his wife without the consent of Chazal.

Mordecai