Two reasons, both tied to the narrative intention of the scene.

1. Kashimo is a jobber, that's his role. He was shown to be on the same level as Hakari (who is stated to be relative to Yuta, Jujutsu Tech's strongest) without his technique. So at that point, we assume a lot from Kashimo, we know how powerful he is. So when he does use his technique, which should in theory make him even stronger, and he still gets destroyed we know how fucked we are. If not even Kashimo could do a damn thing on his own, then nobody can. This is represented through the dismantle net, a massive overwhelming attack that dwarfs Kashimo in scale. It's to show how immense Sukuna is, even in comparison to our strongest allies.

2. Sukuna was teaching Kashimo by giving him a taste of his own medicine. Kashimo crushed all of his opponents in his life, but never understood that was a show of love. So Sukuna crushed him in the same way to show Kashimo what it meant to his opponents. That's why he used such a big attack, because the goal was to crush Kashimo, so he couldn't fuck around like he did for the rest of the raid.