User blog comment:Xilinoc/Ch. 681 - THE END TWO WORLD/@comment-24769684-20160707113033/@comment-1351982-20160707113652

Because then the story would be over. But if he did that then Tsukishima would be way op. He is still human afterall. Yhwach is supposed to be the most powerful being in the Bleach universe at the moment, so I doubt Tsukishima's spiritual powers would be able to overpower him (remember what Aizen told Sui Feng in the Arrancar arc). Being able to fix a sword is a bit more feasible than rewriting history to kill what is essentially a god.