The love for BOTW is pretty easy to explain.
For many people, one of the promises of the video game medium, (a "holy grail" i might even say), is the dream of exploring and going on little self-directed adventures, without the game explicitly telling you what to do, or where to go.
YOU look around, YOU see something interesting, YOU "adventure your way to it", YOU solve it, YOU get the rewards. In a single word, the entire game loop is "discovery".
Previous Zelda games (and many adventure / open world games) were sold this promise, but never actually delivered it! They were always filled with tutorials, and NPCs telling you where to go, and NPCs spoiling puzzles for you, etc...
...and worst of all, various types of invisible walls, telling you "hey you came here too early, wait for an NPC to tell you to." Which basically kills all player interest in exploration and discovery.
For many people, their favorite thing in video games is finding secrets. (Why do people love Mario World? The secrets, dummy!! Switch palaces! Secret exits! Star road!) Well, what is a secret, but something the game didn't tell you? And in BOTW, the game doesn't really tell you much at all...therefore almost everything is, arguably, a secret!
Yes, the game has many MANY flaws. However, for many people, by decently delivering this ONE holy grail of the promise of video games, all those flaws are easily forgiven.
Also it's Zelda.
People who don't finish their sentences
I'm a Castlevania superfan! Ask me anything!