An in-universe explanation is given in the second episode of The Legend of Korra, ‘A Leaf in the Wind.’ Korra expresses her frustration with airbending and Tenzin explains that, “often the element that’s the most difficult for the Avatar to master is the one most opposite to the Avatar’s personality.”
From the moment she was introduced Korra was hotheaded and raring for a fight. She didn’t take “no” for an answer and was more likely to punch her way through a problem than talk it out. It makes sense the other elements were relatively easy for her while airbending, an art that focuses on indirect styles of combat, didn’t come to her as quickly.
Aang had similar issues when it came to earthbending since it was in complete opposition to his personality. Aang wasn’t like Korra, he didn’t tackle problems head-on. Especially in the first season he often solved problems with a calm air and sometimes a joke in tow.
This leads to the second answer to the main question we’re discussing, one that made it so Avatar: The Last Airbender and The Legend of Korra wouldn’t be too similar. Seeing Korra learn all the other elements, as Aang did in ATLA, would have been repetitive. The shows creative staff wisely realized that by aging Korra up from where Aang started and making her naturally talented at the three elements he had to learn, they wouldn’t be giving the audience the same stories they’d already seen. It would allow them to show the process of learning air bending, a process we had seen little of in ATLA.
It also helped inform Korra’s character. When crafting the sequel series the team behind it didn’t just want to repeat ATLA beat for beat. They wanted something new and the idea of a hotheaded Avatar contrasted nicely to what the audience had seen with Aang.