"Pump Up The Volume" was the right record at the right time. For one, there's something youthful and fun that speaks to young listeners with a line like "Pump Up The Volume". This was also at the time of your neighborhood boomboxes, and fit the attitudes of those living in big cities, regardless of race/ethnicity. Lastly, and just as important, Dance music was perceived as cool by Blacks and Latinos in the 80s. While in the first half of the 80s CHR was rediscovering and heavily focusing on pure Pop records, in the second half of the 80s a lot of stations tried sounding a lot hipper by embracing Rap in 1986 (Run DMC vs. Aerosmith, LL Cool, Beastie Boys), and both Dance music and Hard Rock starting in 1987.
Probably more shocking was the fact that Rap first broke on CHR radio in the last few months of the 1970s, though only by a handful of true CHRs, including Y-100 Miami and CKGM 98 Montreal. There were also stations between 1980-85 that weren't afraid to touch it, like Kiss 108 Boston, KRLY 94 Houston, I-95 Miami, and of course Z-100 New York (Scott Shannon embraced records like "White Lines" by Grandmaster Flash).