Mating patterns and genetic superiority
Darwin thought quite a bit about this. He even said "The sight of a peacock makes me sick". https://www.ncbi.nlm.nih.gov/pubmed/10824193
He added to his ideas about natural selection to include sexual selection. Traits selected by females, even when they are non-adaptive for males, will increase the males fitness (as defined by his number of offspring). There is also the idea that perhaps possessing such an outrageous and non-adaptive tail proves that you a good mate, the same way we would think that a guy who could win a sprint while carrying barbells must be superior.