The NAACP reacted to the blaxplotation films by condemning the films’ portrayal of African Americans. Other groups that came out against the films were the Congress of Racial Equality (CORE) and the Southern Christian Leadership Conference (SCLC). Those three groups formed the Coalition Against Blaxploitation (CAB) in 1972 aimed at effecting positive change regarding the roles of African Americans in films.
A shorter answer
They opposed them because the films reinforced negative stereotypes about blacks.
