Why Wasn't There an Oscar Nomination for Best Score?

Yeah, maybe cause Enema, oh excuse me Eminem is white.

Definitely not right, Superfly should have gotten a nomination by the AMPAS.

From the American Film Institute Catalog:
In January 1973, trade papers reported that although it previously had been announced as under consideration as a nominee, Mayfield's song "Freddie's Dead" had been ruled ineligible for entry in the Best Original Song category because it appeared in the film only as an instrumental. [The soundtrack album did feature the lyrics written for the song by Mayfield.] After the announcement, Warner Bros. issued an apology for its erroneous submission of the song for consideration. Controversy continued to swirl around the Academy's decision to exclude the song, with the executive committee of the music branch convening to discuss the song's ineligibility. According to a February 5, 1973 Daily Variety article, the meeting was held "because of the emotions involved," and the song's exclusion was upheld.
On February 23, 1973, Daily Variety related that Mayfield had filed a letter of complaint with the Academy over its method of selecting nominees. The article noted that Mayfield was not qualified for a nomination for Best Dramatic Score because "five songs needed to be submitted, and Warners only submitted three." Although the article stated that Mayfield's manager was contemplating filing suit against the Academy, the action was not taken. It was also noted that Mayfield had accused the Academy of a racial bias, although he was declining to file suit because the title song from Super Fly had been considered for the Oscar balloting.