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The sadly, very easy answer is: "The Walking Dead" is horror.
Although we have made great strides in making other genres more acceptable by the general public, (think fantasy, like "Game of Thrones", or espionage, like "Homeland", or even, *shudder*, romance, like "Outlander",) other genres, (especially horror, but also science fiction,) are still thought of as somehow 'lesser' by people, so of course anything that falls into a ghettoized genre can't be any good to them, whether they've actually watched it or not.
This has been the fight of my life, trying to show people that incredible art exists in all genres, and that horror exists in all other genres, all the time. At any rate, I'll save the rest of the soapbox rant for another day, but that is the answer to your question.
Annual How Come Melissa McBride And Andrew Lincoln Were Subbed By The Emmys post.