While the classic species of Tribble has been a mainstay throughout much of the series, a Star Trek episode focused solely around the Tribbles hasn't been released since Deep Space Nine - why is that? There are likely a few elements involved that can explain why Tribbles have lost their prominence, including the reception, theming, and even select actors' opinions on their involvement. While they are one of the most memorable species, known even to those who don't watch Star Trek, the Tribbles not being used to their full potential actually makes sense in the context of modern Trek.
On December 26, 1967 Star Trek: The Original Series season 2 episode 15 "The Trouble With Tribbles" aired and sparked a legacy. With TOS always trying to conserve costs, the Tribbles populating Star Trek's Enterprise was a relatively easy plot device to accomplish due to their puffball shape. It became one of the most highly rates episodes of the series and garnered a cult following, which inspired Deep Space Nine to not only repeat the success nearly 30 years later but film a crossover episode. "Trials and Tribble-ations" is episode 6 in season 5 of DS9 and depicts the main cast traveling back in time to intercept the crew of the USS Enterprise NCC-1701 before an explosive Tribble planted by a Klingon destroys the past.
Tribbles remaining a fan favorite alien species in Star Trek raises questions as to why they aren't often used. Around when DS9 aired, Star Trek as a franchise was taking a shift in tone. Its creator Gene Roddenberry passed away and Rick Berman had taken up the helm to explore bold new worlds and ideas. Despite this, much of The Next Generation's and The Original Series' moments of levity alongside Roddenberry's trademark conviviality lingered and allowed room for less dire episodes sprinkled in between serious plots, such as "Trials and Tribble-ations." The main reason Tribbles haven't received a mainstream episode centered solely around them since DS9 is because Star Trek has grown so far apart from its original tone. The shows directly following DS9, Voyager and Enterprise proved the franchise was moving into darker territories with themes often revolving around war, politics, and violence. Star Trek's Worf actor Michael Dorn has stated his dislike for Tribbles, claiming they're "just this whole cute thing. It drives me up the wall," which represents how a few of the actors might not have taken the alien species seriously enough for the creators to give them longevity.
In a 2004 treatment for a Star Trek: The Original Series reboot written by J. Michael Straczynski and Bryce Zabel, the Tribbles were to be depicted with teeth and a potentially malicious agenda. This further shows a modern reluctance to revive Tribbles as the gentle and unassuming species they were originally created to be. While there has been an episode of Star Trek: Short Treks based on the Tribbles, they haven't yet starred in any new episodic Star Trek show. Short Trek's "The Trouble With Edward" is set within the Discovery universe but separated from its central narrative, because the comical tone and characters would not fit into the highly intensive action-drama story Discovery portrays. Tribbles could make a huge comeback if Star Trek regains its lighter energy. For now, it appears the franchise is focusing on more gruesome avenues it's rarely explored.
Deep Space Nine's "Trials and Tribble-ations" broke new ground by mixing classic characters with newer ones in a meticulously crafted crossover that managed to pay homage to one of the most memorable Star Trek episodes. Tribbles have become a staple in the franchise, and though they receive some screen time, they still deserve to have another comeback on the same level. A new Star Trek show will need to make inherent modifications to the franchise's overarching darker themes in order for a Tribble-centric episode to be feasible again, but it could happen given enough time for change.