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It's the narrative problem.
The kienzan is his most dangerous attack, but if it hits, it's game over for his opponents. So, in order to progress the story, it's usually avoided. Or if it hits, it's against someone who can regenerate.

The exceptions are glorious, (looking at you Nappa)

The solar flare, on the other hand, decides nothing. And thus, can be used repeatedly without damaging the narrative.

Unlike the spirit bomb, special beam cannon, big bang attack, and kamehameha, the kienzan doesn't take a long time to reach it's full potential. As far as I'm aware at least. Even thte mafuba usually comes with the death of the user at the end of it.

The fact that in this story, the kienzan doesn't feel useless, like it was in the original manga (at best it cut of Frieza's tail) is a testament to Chewys's skill as a writer and storyteller.