In the late 1960s, there was the start of a generational shift in the comic book industry. In the 25 or so years that comic books had been around, the writers had remained mostly steady. Comics were written by people who either started working in comic books as they were invented, or writers who worked in science fiction or the pulps before comic books began. In other words, none of the people who were writing comic books were actually fans of comic books, since they were already adults when comic books began.
The late 1960s, though, began to see the first comic book writers who grew up on comic books. Probably the most famous example of this trend was Roy Thomas, hired by Stan Lee to basically be Marvel's other writer. Thomas had famously done his own comic book fanzine, Alter Ego, before working in comics. Thomas' hire soon led to a groundswell of young writers working at Marvel Comics in the late 1960s and early 1970s.
Meanwhile, over at DC Comics, the changeover was a bit slower in the making. Two writers, though, that tried to change things were best friends Len Wein and Marv Wolfman, who began taking the DC Comics weekly tour of their offices every week in the mid-1960s, when they were still teenagers. They kept pushing to get writing assignments, and eventually Joe Orlando hired the pair to write an issue of Teen Titans in 1968 for editor Dick Giordano.
Dick Giordano had only recently been brought over to DC from Charlton Comics on the recommendation of Steve Ditko, so Giordano ("only" in his late-30s, so practically a baby by DC Comics editorial standards of the time) was more receptive to new talent than some of DC's other editors. In their first issue, Wein and Wolfman introduced a Russian superhero, Red Star, which was quite a rarity at the time...
Basically, what you had here were two progressive 20-year old guys who wanted to try to change the world, and an editor in Dick Giordano who was not opposed to the notion. Once Red Star was approved, Wein and Wolfman decided to follow up by introducing a black superhero in the pages of Teen Titans #20.