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I read all the fanfics on ao3 about this ship, and I read every analysis about this ship. The thing is that despite how cool and smug they are when they both are in battle, at some points, their combat was not different from that between Kazama and Saitou in Saitou's route. Just erase how Kazama tried to humiliate Saitou and think about the battle between Kazama and Saitou again. It's only that none of Kazama or Saitou took time acting as socially as Harada or Shiranui do. Otherwise, some people would ship Kazama and Saitou now as well.
If you look closely, the feelings between Harada and Shiranui were not even close to the feelings between Kazama and Hijikata, which are quite iconic. The battle between Kazama and Hijikata was one of the most iconic things in the whole hakuouki game.
I don't know about you, but as for some other hakuouki fans, the thing is that Harada had an extremely failed route in their opinions, and fans disliked his relationship with Chizuru. His relationship with Shinpachi was a very failed one, too. His relationship with Kondo and how cold he reacted after Kondo's death made fans mad just like how his relationship with Chizuru and Shinpachi made them mad. Lots of fans see Harada as a selfish man. That's why fans ship Harada with Shiranui in hopes of creating a good ship.
Plus, if you pay attention, then you'll notice that it's only Shiranui who was curious about Harada and interested in interacting more with him, but Harada was not really interested. He didn't even find Shiranui as interesting as he found Shinpachi. It's mostly Shiranui who makes everything cool, and Harada is just normal.
The hakuouki team put Shiranui in Harada's route, mainly to make up for how fans were going to dislike Harada and think that he is selfish. After all Shiranui was a guy who lots of fans wished if he had a route with Chizuru or if he just took Chizuru by force from Harada for once and all even if by shooting her with his own gun. Some fans who hated Kazama for attempting to kidnap Chizuru wished if Shiranui took Chizuru by extreme force only because they hated Harada's relationship with Chizuru more.
Have you noticed how there are very few Hijikata x Kazama fanfics and fanarts despite the fact that Kazama chose to be banished from his whole clan and lose his leadership of Oni and lose his statu as the most powerful Oni in the west only and only to fight Hijikata one last time? It's because the hakuouki team put so much more effort on Hijikata x Chizuru and Kazama x Chizuru to write their relationships
Harada and Shiranui interactions are nothing close to Hijikata and Kazama interactions, yet they have more fanfics. Only because Harada's route was a big failure in the eyes of many fans who like Harada.