The lore of Avatar: The Last Airbender kept getting richer and more nuanced as it continued to expand beyond the original show. Some of the details of the world exist solely in the background, however, and since they're never brought up, they're easy to miss. Case in point: Lightning bending's history is one of the richest thematic elements in Avatar's story, but it's mostly relegated to the background.
Most fans are likely familiar with the broad strokes. lightning bending was introduced in the original series as a unique ability that made Azula so dangerous. Uncle Iroh personally developed a technique to redirect lightning by studying the techniques of water benders. By the time of The Legend of Korra, both lightning generation and lightning redirection were more commonplace. But there's still much more to the unique ability.
To go back to the earliest days of lightning bending in the world of Avatar, one would need to turn to the prequel novel The Rise of Kyoshi. Aang's predecessor, Avatar Kyoshi, is only a teenager in the setting of the novel, and through much of the story, there is no mention of lightning bending. However, there is a bandit uprising headed by a man named Xu Ping, and at a pivotal point in the story Kyoshi challenges Xu Ping to a duel to end his reign of terror. To her surprise, Xu Ping does not just strike like lightning, he strikes with lightning.
It turns out that lightning bending was barely known by anyone of Kyoshi's time. In fact, most of the citizens of the Fire Nation who were even aware of it dismissed it as mythic stories from the ancients, but it turns out the Xu Ping was one of the few inheritors of the guarded knowledge. When the novel's villain, Jianzhu, first defeated Xu Ping prior to the novel's story, he only kept Xu Ping alive in prison so his allies in the Fire Nation could learn his secrets.
With Xu Ping's death, it seems that the only way the Fire Nation could have learned of the technique was to have extracted it at some point from their prisoner. Given that nothing else is known about the technique until it pops up again in the original series, it seems that the Fire Nation royal family held onto the secrets covetously.
The only lightning benders throughout Avatar: The Last Airbender are Ozai, Azula and Iroh, all of whom were members of the Fire Nation royal family. The ability is said to be difficult to master, and indeed even the highly talented Zuko proves incapable of producing even a spark, but there are other fire benders who appear who never demonstrate the ability themselves. Even Jeong Jeong, Aang's first fire bending master, never demonstrates the ability.
The Legend of Korra ends up explaining the mystery of lightning bending in the most subtle way possible. Taking place decades after the original series, Korra shows a world thriving off the technological advancements of their era. Electricity is extremely commonplace, and only a blink-and-you-miss-it detail explains why: Lightning benders work at factories fueling Republic City's generators. The once-exclusive bending art is widely available to the people, so what changed in the interim?
It would be lazy to just call the detail an inconsistency on the show's part, as there's clearly a lot more to it than that. The idea is secretly genius as it points out just how much the world could thrive off of the knowledge that Fire Lord Zuko made publicly available after he was crowned. Rather than being fascistic and openly opposing the free exchange of ideas like they had for centuries prior, Zuko creates an era in the Fire Nation where knowledge is democratized to the benefit of all.
The trajectory to the history of lightning bending ends up serving as a beautiful thematic piece to Avatar's world. When the knowledge was rarest is when it was the most dangerous, serving only as a weapon wielded by a power-thirsty criminal. When the knowledge was allowed to intermingle with other philosophies through Iroh, the weapon became a way to save lives. Finally, once the knowledge was made openly available, it benefited the entire world and wrought a new age of technological prosperity.