Why are we labelling everything as 'Dark'?

My main frustration with modern literature is the advertising that serves as clickbait for those who might want to be considered 'niche' but aren't necessarily looking for anything 'niche'.
Why is it a 'dark' romance when everything has a positive turn around? Why is it a 'dark' fantasy when it's just a beauty and the beast retelling in a fairy world? Alice in Wonderland retellings aren't dark....in my opinion.

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