The same reason Kenpachi gets time in the lime light, but isn't exactly a main character, he's a character in this story for Ichigo to directly compare to. I never said he'd not get times to shine, like Kenpachi gets his. I was going to say, he'd directly fight Gin but lose, but I wanted Byakuya to rely on him in the second arc. As a stronger companion, but I also wanted Byakuya and Ichigo to be mainly even. If I had to write his times to shine, he'd fight Hollowfied Komamura to a stand still (bc I mean, he's facing an army of Komamuras). And, instead of Yama and Ichigo directly going into a fight, Ichigo fights Aizen to an almost immediate defeat, where Aizen admires Ichigos strength, and grows closer with him as a result, and due to this, Aizen is the one giving Ichigo the courage to defeat Yhwach.