They’ve been on Black People heads since ragtime. Doesn’t matter.It all goes back to Jim Crow. Black people(east of the Mississippi) have had a longer history of propaganda against them and have lived next to whites the longest. Blacks have a longer history of being seen as the menacing "other". Although this is changing quickly with trump and other conservatives zeroing in on Hispanic migration. MS13 has been hyped up as one of the most violent gangs since I was a kid..
Also gangster rap. Let's be real, the various genres of music from Latin America don't have "menacing" or "scary" attached to them the same way gangster rap does. Salsa, merengue, mariachi doesn't have the same connotation as trap music does. And trap/gangsta has been the most commercially successful form of black music for the last 30yrs.

)people are seen as the least threatening.