Well I can see why this is popular. When you so perfectly combine lifelong friendship and post-war Italy you can't miss.

Does everyone have the brilliant friend? The friend who you share a unique understanding of the world with that no one else seems to grasp? The friend that makes you feel both less and more than what you are?

That's nice to know, it's a friendship you don't see in literature enough.

Also, surely I'm not the only one that absolutely loved when Ferrante did that thing that some novels do, where it turns out the brilliant friend in the title wasn't who you expected. As the Elena of my duo, I appreciated the ego stroke.