@Background Pony
Numerous reasons I suspect.
A. She is the ultimate authority figure. And some people just hate authority.
B. Many people feel since they like Luna, they have to hate Celestia. It’s the same thing that happens with Batman and Superman. Many people latch onto Batman because he’s dark and edgy, and consider Superman Boring.
C. She may have the most wasted potential of any character in the show. How much do we really know about her?
D. How she is portrayed leaves a lot of unfortunate implications. She sends six young mares off to fight every single evil creature that comes her way. She has pretty much molded Twilight into her heir of sorts. She ascended Twilight without so much as even asking her. She treated Sombra’s threat to the empire like a test. She invited the ponies to the gala because she found it boring. She has a trolling streak. It often seems for such a wise ruler that she does not think things through.
Take your pick.