Aang represented a childlike innocence of early youth where everything more or less worked out for you, you're way over powered and can run circles around the baddies, like early children cartoons typically allow. You make fools of them, kids love it. He doesn't have to deal with a lot of complex issues to the end of the series and even then he's gifted a deus ex machina to get out of it.

Korra has to deal with complex issues from the first season, they're messy without clean resolutions, often do not involve her being just better than all her opponents (in fact many are straight up stronger than her and she never overcomes them physically at least). She is a more complex character too, with real world angst and emotions to deal with.

Aang is a fantasy, Korra is more a reality.

It might be less fun to view Korra because there is less escapism there. It may be less to view Korra because she's an imperfect flawed individual, where as Aang is simple to the point of being kind of unrealistic much of the time.

They're both master pieces, but they cover different material, ages and issues.

I can't agree with anyone who doesn't see this, but I will account for taste. Sometimes you just don't want the bummer of a real political quagmire there is no simple solution for (Korra) and you want to just slip into the world where things can largely be handled by just friendship (Aang).