For decades, many Star Trek series have followed the tried and true "alien of the week" format. That generally meant introducing a new species or group every few weeks for the past several years. The modern franchise frequently falls back on the old favorites, but the creativity of constantly introducing new races kept things fresh. Look at the Kazon, the aliens based on Los Angeles gang culture.
Everyone can name a few notable Star Trek races. The Klingons, the Romulans, the Vulcans, a stranger on the street could probably pull one of those names from memory. For every particularly memorable alien species, dozens are introduced and tossed aside instantly. They deserve some attention, for better and for worse.
What are the Kazon?
The Kazon were introduced in Star Trek: Voyager as one of the series' primary antagonists. Their unnamed homeworld was conquered some time ago by another race called the Trabe. The Trabe held the Kazon in bondage for generations. The Trabe introduced various methods of keeping them under control. Their most effective was dividing the Kazon into sects. This allowed the Trabe to keep the Kazon fighting for survival instead of rising up against their oppressors. In 2346, a lone adult Kazon unified the sects in violent rebellion against the Trabe. Their uprising was successful, forcing the Trabe to flee the planet and never settle on another one. Unfortunately, their sects remained intact and continued to fight.
Kazon are patriarchal, violent, and primitive. They raise their male children to be warriors by birthright. They treat women of their species as second-class citizens. They have access to some technology but are still scientifically decades behind Starfleet. There were seven main sects of the Kazon, two of which possessed the lion's share of the species' power. Though they were enslaved for an unknown period, they freely enslaved others. They're introduced holding a woman in bondage to steal the resources of her people. They intend to steal the technological advancements of the Voyager to further their conquest of the galaxy. They strand the crew of the Voyager on a desolate planet. They only give up after an explosion kills a few of them. The Kazon are the only species the Borg Collective considers "unworthy of assimilation." They're terrible, cruel beings who willingly fight each other and everyone else for possession of the entire quadrant. They didn't last very long.
Why aren't the Kazon around anymore?
The Kazon largely disappeared after Star Trek: Voyager season 2. They're frequently listed among the worst villains of the entire Star Trek canon. Fans, critics, and crew members had a laundry list of problems with the Kazon. They appeared too frequently, giving a false impression that their empire stretched beyond that of any other force in the universe. The cast of Voyager dunked on the Kazon left and right, comparing them unfavorably to every other notable antagonistic species. Speaking of comparisons, everyone involved described the Kazon as cheap knock-offs of the series' most popular alien race, the Klingons. The Kazon were introduced alongside the Sikarians and the Vidiians. Both of the other options were seen as superior to the Kazon. The species raised complaints from everyone involved, many of which are still heard loud and clear. Perhaps worst of all, the Kazon raised a lot of unpleasant questions about the nature of the series.
The Kazon were notable for being enslaved. That's the kind of thing that might engender sympathy if the race weren't so unforgivable. They enslave others the instant they have the power to do so. They're violent, cruel, hateful, sexist, and above all else, primitive. They're constantly fighting each other, but everyone else is terrified of them. They're explicitly a metaphor for gang culture. The two main groups are naked metaphors for the Crips and the Bloods. Their dark skin and bushy hair aren't exactly subtle. Many viewers and critics pointed out the implicit racism in the design of the Kazon. Worse yet, they fit into a larger trend in the franchise using dark-skinned races to represent the worst aspects of human behavior. Unlike all the fair-skinned races in the galaxy, there's no reasoning with the Kazon. All they know is violence. They're criminals and murderers from the moment they escaped slavery. A moment of critical thinking applied to this concept reveals why the Kazon don't pop up anymore.
Star Trek has been around for a long time and made a lot of mistakes. Some ideas escape the writer's room before their time and live on as an embarrassment for the crew. People were understandably quick to condemn the Kazon. When the cast, crew, critics, and fans turn against something, it's rarely worth keeping around. The Kazon should be a lesson to all science fiction authors. Be mindful of the implications of every choice, lest people accurately decode the violently unsubtle metaphor and see the unpleasantness underneath.