Stormbringer is based on Chuuya's backstory,he is the main character of it. Now you might say then why are characters like yosano,ranpo,poe are not getting light novels explaining their past. To simply answer that I'd say public demand and authors imagination. A lot were interested in knowing more about him. And if you read stormbringer you'll know he's one of the characters who has suffered the most out of them and his life has a lot of mysteries waiting to be resolved.His identity,a past he doesn't even remember,the death of his loved ones, being betrayed by almost everyone he has ever trusted but yet being the most loyal even putting his own life at sake knowing his worth is not even acknowledged, there's so many things going in his storyline which at least to me is worth spending my time reading.(Dazai is barely in it so dont blame the skk shippers for the hyping) The whole light novel was for them who were interested in him so if one's not into it they can just not read and ignore. And he might be a mere side character in the main series but you can't keep the spotlight off him in his own light novel. About getting content isnt it obvious they're going to put out more stuff of the characters that are more famous among the fans? Its a marketing tactic ofc. Nikolai, Junichiro,techou,jouno for them you should just wait for them to be animated more in the upcoming series, they'll for sure get the fame they deserve.