Why is Avatar Korra Less Popular than Avatar Aang?

In terms of story, Korra is not as epic as Aang.

In Aang's plot, from the beginning, it was prepared that his main antagonist, his arch-nemesis, was the Fire Lord. 

Although Zuko was also present, since season 1 we could already see that there was a dilemma within Zuko.

This is not present in Korra's plot. 

There is no grand enemy or arch-nemesis in Korra's plot, instead, each season has different antagonists. 

In season 1 there is Amon and the anti-bending movement, in season 2 there is the Dark Avatar, in season 3 Korra faces the Red Lotus, and in season 4 there is Kuvira.

It can be said that the arch-nemesis for Korra is Korra herself, in the sense that her brash, impatient, and head-on nature is her own enemy. 

It is similar to Aang's character development from a childish figure to a mature one, but Korra's development is not as good as Aang's. 

While Aang develops by discovering new powers and learning to control them, Korra's development is from someone who is already skilled and then finds maturity through losing her powers.

The placement of the antagonists is also not appropriate, in my opinion. 

Placing Kuvira as an antagonist after Zaheer made the tension in the story decrease. 

After Korra found a very strong enemy in Zaheer (who can fly without assistance, what's less powerful than that?), the next threat came, which relied only on technology in the form of giant robots, making the plot less thrilling.

Other shortcomings, in my opinion, are:

• There is no extraordinary threat like the threat of Sozin's Comet in Korra's story.
• There is no character that finds redemption like Zuko.
• The political intrigue is not as "delicious" as in Aang's story, there is no Long Feng's betrayal, no Azula's deception, it tends to be flat.
• There is no development of side characters, no story like Katara training with Pakku or Sokka with Piandao.
• Bolin's role as comic relief is not as good as Sokka's.