Because its more sensational news. Thank your media. Just like we are conditioned to believe there are more white serial killers, but that is not the case....it media conditioning.

"In the United States, the majority of reported and investigated serial killers are white males, from a lower-to-middle-class background, usually in their late twenties to early thirties.[6][12][13] However, there are African American, Asian, and Hispanic (of any race) serial killers as well, and, according to the FBI, based on percentages of the U.S. population, whites are not more likely than other races to be serial killers.[13] Criminal profiler Pat Brown says serial killers are usually reported as white because the media typically focuses on "All-American" white and pretty female victims who were the targets of white male offenders, that crimes among minority offenders in urban communities, where crime rates are higher, are under-investigated, and that minority serial killers likely exist at the same ratios as white serial killers for the population. She believes that the myth that serial killers are always white might have become "truth" in some research fields due to the over-reporting of white serial killers in the media."

[link to en.wikipedia.org]