The penultimate arc of Dragon Ball Z introduced the villainous Majin Buu, a pink-colored enemy with as many forms as he had powers. The Z-Fighters had to give the fight with Buu their all to put a stop to him, but in the end, it still came down to Goku and Vegeta saving the day once again.
Part of the early focus of the Buu Saga was the new concept of the Fusion technique, which manifests initially in Goten and Trunks being trained to combine. Unfortunately, their defeat and the focus returning to the old protagonists made this seem almost superfluous in the long run. Given how little it mattered in the end, it appears that Fusion was emphasized only to become a side story in the Goku and Vegeta show.
Dragon Ball's Focus on Fusion
The introduction of the Fusion technique comes almost out of nowhere, and it's blatantly an example of shonen power scaling. The method is taught to the young Trunks and Goten as a secret weapon against the rising threat of Majin Buu. Getting the technique just right takes a while for the two to master, resulting in some comical combinations that aren't quite at full power. Once they get the hang of it, however, Gotenks proves to be a powerful young fighter. He even achieves the Super Saiyan 3 form with extreme ease, which is bolstered by Vegeta not achieving the transformation.
Eventually, however, Gotenks' arrogance and Super Buu's power prove to be too much to handle, with Majin Buu felling the fighter and consuming him with his Candy transformation ability. This brings Trunks and Goten's mutual time in the spotlight to an abrupt end, with only Trunks regaining it in the now non-canon Dragon Ball GT. Their brief emphasis was part of Toriyama's wishes for Goku to no longer be the franchise's protagonist, but when that fell through, all of the payoffs of focusing on the next generation faltered with it.
The Failure of Fusion in Dragon Ball
Hindsight makes the focus on Fusion seem like a complete waste. Sure, Gotenks put up a good fight against Super Buu, and the same could be said for Vegito once Goku and Vegeta fused. Despite this, the conclusion still involved a separated Goku and Vegeta saving the day against the bad guy without the technique, making its usage theretofore seem like mere window dressing or narrative padding to draw out the fight against Majin Buu.
Gotenks never reappeared in GT, and his roles in Dragon Ball Super have been incredibly lackluster. When the adult Gotenks finally shows up in the Duplicate Vegeta arc, he doesn't make much of a dent against the villain before being taken out and defused. Goku and Vegeta's other fused form of Gogeta only came into official canon recently, with his appearances before being relegated to non-canon sequels and movies. His canon debut does make him seem a bit more substantial than Goten and Trunks' combined form, but that's part of the problem.
The Fusion Technique was introduced and first taught to Goten and Trunks, and Gotenks arguably had more focus in terms of screentime than Vegito did. Despite this, his appearances almost all involve him eventually being beaten by the villain and never truly getting any wins. It's telling that the only Fusion forms with any real success involve Goku and Vegeta, which sadly proves that the false hope of the next generation of Z-Fighters being of any real prominence was as much of a waste as focusing on Gotenks.