Why are the Iluminati in rap lyrics?

Beyonce, who says she's not a member of the Illuminati. “Y’all haters corny with that Illuminati mess.”

One reason that Illuminatus! remains relevant, in spite of the fact that it's also a product of a particular time, is that interest in the Illuminati has not faded. 

"Why Is Rap So Obsessed With the Illuminati?" by Steven J. Horowitz traces the first mention of the Illuminati in rap to a 1995 song, a 1995 remix of LL Cool J's "I Shot Ya." 

"It was the beginning of an entirely new school of thought in hip-hop, one as intelligent and informed as it was suspicious and paranoid," Horowitz avers.

Informed? Maybe. I like the bit about Jay-Z wearing the "Do What Thou Wilt" sweatshirt.