As far as I can tell, people's love for it is usually related to one's tolerance for sincerity. We'll watch and read and play lots of works about love and family but it'll be wrapped up with war or drama or political debates and stuff. It's rare that it's the unabashed point of an entire work, with quite a bit of subtly removed, let alone such a long work.
That turns off a lot of people. If this poll is any indication, it's a good trait for most viewers. Anyone can acknowledge the animation and sound is good but...that's never really enough for an animation to build a fanbase. Having a cartoon (that's from the west) thoroughly based in character exploration and relationships, and what relationships mean and how/why they're formed and how they are broken....that's not just rare, it hasn't really...happened before.
The show also has a lot of subtly, particularly the villains and the very symbolic meaning of their powers....And some mild subtext about gender, though it's rarely emphasized.
...I dunno, if the world doesn't end from this notion I might talk about it in greater detail someday.