For that matter, why haven't the windigoes shown up to feed off the humans' misery/hatred?
Idiot Ball: Many readers felt The Reveal in Chapter 15 with regards to why the humans were enslaved was a case of Celestia holding a particularly large one of these.
Too Bleak, Stopped Caring: The horrible ways some pony slave owners are shown to treat their slaves and Celestia's gross incompetence has caused a lot of readers to either give up on the story altogether, or just continue reading in the hopes of Celestia and all of ponykind getting a real punishment for their actions. Some readers even expressed a desire for the Equestrians to be enslaved as a form of Laser-Guided Karma.
Unintentionally Unsympathetic: Celestia. While she clearly did not enjoy turning the humans into a Slave Race for the ponies, her rather weak attempts to justify her decision to Luna as well as the fact that she refuses to just overturn slavery overnight have made her come off like a weak and incompetent leader easily manipulated by her Evil Chancellors to many a reader.
Celestia: "Luna, please, I need your help. I want your help to bring things to how they should be, with humans free."
Celestia: "That would not solve the problem. Ponies have had humans as slaves for centuries, to just force them to give it up would be difficult, these things will take time, but..."
The fact that it took protests from pegasus abolitionists to outlaw the practice of human gladiatorial rings, as well as the fact that a great many slaves are being mistreated (Twilight and Applejack are very clearly exceptions rather than the rule), yet Celestia doesn't seem to have done much to improve those conditions doesn't exactly help her case either.
