Here is what I have concluded through the years..
It's an awareness and realization that this genre holds the most true and honest talent that can be found. It's popular with musicians who are striving to excel and learn from those who really have talent to learn from and inspire.
These bands care deeply about what they create, how they present and package it.
It's not a generic, prefabricated product. it's real and honest, and musicians work hard to make it enjoyable. These factors create a sort of personal bond between the bands and their fans.
That's how I see it. I think the "intelligence" factor is just bullshit, really. It's people who care to find great talent within a world of much music that is meaningless, boring, and lacks stimulation.