The power plants in Korra are powered by lightning bending and it’s portrayed as dangerous work done by industrial laborers and not royals. I think it’s a safe conclusion to assume that the royals opened lightning bending schools to teach non-royals so they could perform this labor. While lightning bending is depicted as rare in ATLA, I think it’s more due to it being very destructive and the Fire Nation wanting a strict control over the knowledge to protect themselves from insurrection.

Also after the events of the ATLA show, the comics continue the story and Toph opens a metal bending school. Metal bending is portrayed quite frequently in Korra in contrast to ATLA where the only metal bender was Toph (who is the sole metal bender in the world at the time). So I’m not sure if any specialized skill is inherently rare, just controlled by access to knowledge.