The problem is, despite being half demon, the show comes across as a witch story. And with the exception of Bewitched and Charmed, witches have no staying power on television.
The very fact that there are exceptions proves that you can't generalize like that. Most shows of any
genre or subject matter fail. Many police procedurals are hits, but many more are flops. Many medical dramas are hits, but many more are flops. It's simply the nature of television that the majority of shows fail. It can't be blamed on the subject matter, because it's the same for every subject matter.