But FFX is "My Story..." says Tidus.

I would never even consider Shuyin since he is one of the lamest villains in the series. It would be like asking why Cloud isn't fighting Genesis since he's another big villain in FFVII.

Yet, not all the heroes face people they have true antagonistic relations with. Tbh, the Onion Knight should be facing Xande since CoD doesn't even appear until he's dead. From a story perspective though, its a better match up since the Warriors of Light came to be in order to stop CoD. Tidus' major conflict in X was battling his father who had become Sin and lets face it, their relationship is truly antagonistic towards each other. I feel it works in the same way that Cecil and WoL don't square off with their exact final bosses. They chose the villains that had the most significance to the heroes. Seymour was just a nusance to Tidus, he's more of a villian to Yuna, Rikku, and Kimarhi. Besides, we don't need another mage on the Chaos side, even if he is a zombie mage