It's probably because the writing's laziness has showed itself full-front at the start of the Juo arc. We had several instances here and there that made us roll our eyes and accept things - that god-seeking group that somehow never fought strong masks before, Yuri getting a huge power-up and using her powers rather than her brain like she used to, Rika spending many volumes being kidnapped, the backstabbing father, and etc. And I imagine those who kept reading tried to look past all that by enjoying the other parts of the manga, like Sniper Mask being a mysterious badass, Nise and Yuri shipping, or whatnot.
But nothing really set people off quite like the filler chapters we've been getting dozens in. And it just so happens that it's centered on developing Rika, who is one of the least interesting characters when by himself imo. Yuri is pretty mary sue, but at least she has character dynamics with several different people to save her. Even Kuon, as much as people dislike her, has her character interactions with Sniper Mask, Nise, and Yuri to make her bearable sometimes. (Unfortunately, now both don't even get to reap any of those advantages because reasons.) Rika only had Aikawa. No, really. Sure he has Sniper Mask and Yuri, but the manga spent over a dozen volumes limiting interactions between them. Making Sniper Mask Rika's brother doesn't change much, unfortunately. And don't get me started with the supporting characters that Rika had for a team. They've been very minor up until the Juo arc.
Before, the writing would try to keep things interesting by switching perspectives between several different groups, but now it feels like we're just keeping tabs on people rather than seeing the story progress. The writing feels like it's struggling. Ironically, Aikawa's death caused things to spiral and it doesn't feel like the author has a grasp on how to make things interesting again.