Generally, I would imagine it is because he is not considered the "grandfather of hip hop," as he did nothing to develop the break beat style that defines hip-hop music (other than provide some samples for hip hop artists to use). Blowfly was nothing more than a character - author of such "inspiring" tunes as "My Baby Keeps Farting in My Face," "Shitting on the Dock of the Bay," "Spermy Night in Georgia," and "It's a Faggot's World" (I apologize for any offense, but these are actual titles of his music).
If DJ Kool Herc had passed, you would definitely hear of it.