Because she's a woman. It really is that simple.
If you try to get people to explain why they don't like her, a lot of it boils down to her being bossy or not showing enough emotion, which are ways we try to keep women in the kitchen. Other than that, people often can't come up with a reason. "I just don't!" Hence, "but her emails!"
Women are not immune from reacting this way. I, especially when younger, felt the same way about her. But when I actually listened to her and paid attention to her policy ideas, I found someone who is compassionate and loving, though maybe a bit brash. But in a man we'd excuse the brashness, or even welcome it.
So, yeah. I don't like the hate she gets, and I wish many folks would actually listen to her (she's more right-wing than I am, especially in regards to war; I think a lot of the Never Trump Republicans would have voted for her if they had been given a list of policies to vote on without a name attached), as they'd find someone who is quite often what we claim to want in politics. She's not perfect, by any means, and is certainly part of the political machine, but I think when it comes to a president, that's a much more effective option. That said, I do "understand" the hate she gets because it's baked into our culture. It sucks. She doesn't deserve it.
Start over. Plenty of non-self-hating women hate her.