Nobuo Uematsu revealed that he never intended for FFX's "To Zanarkand" to be the theme song for the game. It was actually a song for a flute recital that was unused, but when Uematsu was falling behind deadline, he submitted this for FFX's theme lol
Here's the full translation:
Uematsu: "To Zanarkand" was actually not meant to be the theme for FFX. There was a flute player from France who originally asked me to write a song for them to play during a recital. When I wrote the piece, I thought perhaps this sounded a bit too sad for a recital. So I set it aside for the time being. Sometime after, when I was making the score for FFX, I was falling behind all the while the Producers were hounding me. So since that flute piece was unused, I just kinda meekly handed it to them. They listened to it and said "Yes! This is brilliant!"