Why aren't maggots used to treat necrotizing fasciitis?

Initially. Maggots are not used because prompt surgical debridement is the procedure of choice because this rids the area of necrotic tissue. Maggot debridement would take to long. After the initial insult is over, one could use maggots to clean the wound up.
They ARE used. Maggot therapy is sometimes used to treat necrotizing fasciitis (nf). But, because nf is generally a life- or limb-threatening emergency, immediate surgical intervention is the first or preferred choice. Still, numerous reports in literature (surgery inadequate or vital structures at risk) where maggot debridement therapy (mdt or biosurgery) was used.
Infection. Maggot therapy is useful to clean wounds that are not infected not infected wounds.